\input macros
\drafttrue
\def\bookline{\CLOS\  Specification}
\def\chapline{Functions in the Programmer Interface}

\beginChapter 2.{Common Lisp Object System Specification}%
{Functions in the Programmer Interface}{Functions in the Programmer Interface}

This document was written by Daniel G. Bobrow, Linda G. DeMichiel,\break
Richard P. Gabriel, Sonya E. Keene, Gregor Kiczales, and David A. Moon.

Contributors to this document include Patrick Dussud, Kenneth Kahn,\break
Jim Kempf, Larry Masinter, Mark Stefik,
Daniel L. Weinreb, and Jon L White.

\endTitlePage

\beginSection{Introduction}

This chapter describes the functions, macros, special forms, and
generic functions provided by the \CLOS\ Programmer Interface.  The
Programmer Interface comprises the functions and macros that are
sufficient for writing most object-oriented programs.

This chapter is reference material that requires an understanding of
the basic concepts of the Common Lisp Object System.  The functions
are arranged in alphabetic order for convenient reference.

The description of each function, macro, special form,
and generic function includes its purpose, its syntax, the
semantics of its arguments and returned values, and often an example
and cross-references to related functions.

The syntax description for a function, macro, or special form
describes its parameters. Here is an example of the format for
the syntax description of a function:

\label Syntax:

\Defun {F} {{\it x\/} {\it y\/} {\opt} {\it z\/} \key {\it k\/}}

\noindent This description indicates that the function {\bf F} has two required
parameters, {\it x\/} and {\it y}.  In addition, there is an optional
parameter {\it z\/} and a keyword parameter {\it k}.

The generic functions described in this chapter are all standard
generic functions.  They all use standard method combination.

The description of a generic function includes descriptions of the
methods that are defined on that generic function by the \CLOS.  A
{\bit method signature\/} is used to describe the parameters and
parameter specializers for each method. Here is an example of the
format of a method signature:

\label Method Signature:

\Defmeth {M} {({\it x\/} {\it class}) ({\it y\/} t) \&opt {\it z\/} \&key {\it k\/}}

\noindent This signature indicates that the method {\bf M}
has two required parameters, {\it x\/}, which must be an instance of
the class {\it class}, and {\it y}, which can be any object. In
addition, there is an optional parameter {\it z\/} and a keyword
parameter {\it k}.  This signature also indicates that {\bf M} is a
primary method and has no qualifiers.

The syntax description for a generic function describes the lambda-list
of the generic function itself, while the signatures describe the lambda-lists
of the defined methods.

Any implementation of the \CLOS\ is allowed to provide other methods
on the generic functions described in this chapter.

\vfill\eject
It is useful to categorize the functions and macros according to their
role in this standard:

\Vskip 2pc!
\beginlist
\item{\bull}
{Tools used for simple object-oriented programming}

These tools allow for defining new classes, methods, and generic 
functions, and for making instances.   Some tools used within
method bodies are also listed here.   Some of the macros listed here have 
a corresponding function that performs the same task at a lower level of
abstraction. 

{\bf call-next-method}\hfil\break
{\bf change-class}\hfil\break
{\bf class-changed}\hfil\break
{\bf defclass}\hfil\break
{\bf defgeneric}\hfil\break
{\bf defmethod}\hfil\break
{\bf generic-flet}\hfil\break
{\bf generic-function}\hfil\break
{\bf generic-labels}\hfil\break
{\bf initialize-instance}\hfil\break
{\bf make-instance}\hfil\break
{\bf next-method-p}\hfil\break
{\bf slot-boundp}\hfil\break
{\bf slot-value}\hfil\break
{\bf with-added-methods}\hfil\break
{\bf with-slots}


\item{\bull}
{Functions underlying the commonly-used macros}

{\bf add-method}\hfil\break
{\bf ensure-generic-function}\hfil\break
{\bf get-method}\hfil\break
{\bf make-instances-obsolete}\hfil\break
{\bf no-applicable-method}\hfil\break
{\bf remove-method}\hfil\break
{\bf slot-exists-p}\hfil\break
{\bf slot-makunbound}\hfil\break
{\bf slot-missing}\hfil\break
{\bf slot-unbound}\hfil\break
{\bf update-instance-structure}

\vfill\eject
\item{\bull}
{Tools for declarative method combination}

{\bf define-method-combination}\hfil\break
{\bf invalid-method-error}\hfil\break 
{\bf make-method-call}\hfil\break
{\bf method-combination-error}\hfil\break
{\bf method-qualifiers}

\item{\bull}
{General Common Lisp support tools}

{\bf cboundp}\hfil\break
{\bf class-name}\hfil\break
{\bf class-of}\hfil\break
{\bf cmakunbound}\hfil\break
{\bf describe}\hfil\break
{\bf documentation}\hfil\break
{\bf print-object}\hfil\break 
{\bf symbol-class}\hfil\break
{\bf symbol-macrolet}

\endlist

\vfill
\endSection%{Introduction}

\beginSection{Notation}

We use an extended Backus Normal Form (BNF) to describe the syntax of
the \OS.  In this section we discuss the syntax of BNF
expressions.  The primary extension we use is as follows:

$$\lbrack\!\lbrack\, O\,\rbrack\!\rbrack$$

An expression of this form will appear whenever a list of elements is
to be spliced into a larger structure and the elements can appear in
any order. The symbol $O$ represents a description of the syntax of
some number of syntactic elements to be spliced; that description must
be of the form

$$O\sub 1\ \vert\ \ldots\ \vert\ O\sub n$$

\noindent where each $O\sub i$ can be either of the form $S$ or of
the form $S{\rm *}$.  The expression $\lbrack\!\lbrack
O\,\rbrack\!\rbrack$ means that a list of the form

$$(O\sub{i\sub 1}\ldots O\sub{i\sub j})\quad 1\leq j$$

\noindent is spliced into the enclosing expression where if $n \neq m$
and $1\leq n,m\leq j$,
then either $O\sub{i\sub n}\neq O\sub{i\sub m}$
or $O\sub{i\sub n}$ and $O\sub{i\sub m}$
are each of the form $S{\rm *}$.

For example, the expression

$$(\hbox{{\tt x}}\ {\lbrack\!\lbrack}\,\hbox{{\tt A}}\ 
 \vert\ \hbox{{\tt B}}{\rm *}\ \vert\ \hbox{{\tt C}}\,
  {\rbrack\!\rbrack}\ \hbox{{\tt y}})$$

\noindent means that at most one {\tt A}, any number of {\tt B}'s, and
at most one {\tt C} can occur in any order.
It is a description of any of these:

\screen!
(x y)
(x B A C y)
(x A B B B B B C y)
(x C B A B B B y)
\endscreen!

\noindent but not any of these:

\screen!
(x B B A A C C y)
(x C B C y)
\endscreen!

\noindent In the first case, both {\tt A} and {\tt C} appear too often,
and in the second case {\tt C} appears too often.

\vfill\eject
In order to make this new syntax more readable, we introduce a
simple indirection extension:

$$\downarrow\!O$$

\noindent If $O$ is a non-terminal symbol, then the right-hand side
of its definition is substituted for the entire expression 
$\downarrow\negthinspace O$. For example, the following BNF is equivalent to
the BNF in the previous example:

$$(\hbox{{\tt x}}\ {\lbrack\!\lbrack}\downarrow\!O\,
{\rbrack\!\rbrack}\ \hbox{{\tt y}})$$
$$O::= \hbox{{\tt A}}\  \vert\ \hbox{{\tt B}}{\rm *}\ \vert\ \hbox{{\tt C}}$$

\endSection


\begincom{add-method}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf add-method} adds a method to a generic
function.  It destructively modifies the generic function and returns
the modified generic function as its result.

\label Syntax:

\Defgen {add-method} {generic-function method}

\label Method Signatures:

\Defmeth {add-method} {\vtop{\hbox{({\it generic-function\/} standard-generic-function)}
\hbox{({\it method\/} standard-method)}}}

\label Arguments:

The {\it generic-function\/} argument is a generic function
object.

The {\it method\/} argument is a method object.  The lambda-list of
the method function must be congruent with the 
lambda-list of the generic function, or else an error is signaled.
If the generic function has no lambda-list, an error is signaled.

\label Values:

The modified generic function is returned.  The result of {\bf add-method} 
is {\bf eq} to the {\it generic-function\/} argument.

\label Remarks:

If the given method is already one of the methods on the generic
function, as determined by {\bf get-method}, or if the method object
is a method object of another generic function, an error is signaled.

\label See Also:

{\bf defmethod

defgeneric

get-method}

\endcom




\begincom{call-next-method}\ftype{Function}

\label Purpose:

The function {\bf call-next-method} can be used within the body of a
method defined by a method-defining form to call the next method.

The function {\bf call-next-method} returns the value or values
returned by the method it calls.  If there is no next method, an error
is signaled.

The type of method combination used determines which 
methods can invoke {\bf call-next-method}.  The standard method
combination type allows {\bf call-next-method} to be used within primary
methods and {\bf :around} methods.  The standard method combination
type defines the next method as follows:

\beginlist
\item{\bull}
If {\bf call-next-method} is used in an {\bf :around} method,
the next method is the next most specific {\bf :around} method, if one is
applicable.

\item{\bull}
If there are no {\bf :around} methods at all or if {\bf
call-next-method} is called by the least specific {\bf :around}
method,  other methods are called as follows:

\itemitem{--} All the {\bf :before} methods are called, in
most specific first order.  The function {\bf call-next-method}
cannot be used in {\bf :before} methods.

\itemitem{--} 
The most specific primary method is called.  Inside the body of a
primary method, {\bf call-next-method} may be used to pass control
to the next most specific primary method.  An error is signaled if {\bf
call-next-method} is used and there is no applicable primary method.

\itemitem{--} All the {\bf :after} methods are called in
most specific last order.  The function {\bf call-next-method}
cannot be used in {\bf :after} methods.

\endlist

For further discussion of {\bf call-next-method}, see the sections
``Standard Method Combination'' and ``Built-in Method Combination
Types.''

\label Syntax:

\Defun {call-next-method} {{\rest} args}

\label Arguments:

When {\bf call-next-method} is called with no arguments, it passes the
current method's original arguments to the next method.  Neither
argument defaulting, nor using {\bf setq}, nor rebinding variables
with the same names as parameters of the method affects the values
{\bf call-next-method} passes to the method it calls.

When {\bf call-next-method} is called with arguments, the next method
is called with those arguments.  When providing arguments to {\bf
call-next-method}, the following rule must be satisfied or an error is
signaled: The ordered set of methods applicable for a changed set of
arguments for {\bf call-next-method} must be the same as the ordered set of
applicable methods for the original arguments to the generic function.
Optimizations of the error checking are possible but they must be
invisible to the programmer.

If {\bf call-next-method} is called with arguments but omits
optional arguments, the next method called defaults those arguments.


\label Values:

The function {\bf call-next-method} returns the value or values
returned by the method it calls.  

\label Remarks:

Further computation is possible after {\bf call-next-method} returns.

The function {\bf call-next-method} has lexical scope and indefinite extent.
%The lexical scope includes any default initial value forms that appear in
%the method's lambda-list.

For generic functions using a type of method combination defined by
the short form of {\bf define-method-combination}, {\bf
call-next-method} can be used in {\bf :around} methods only.

The function {\bf next-method-p} can be used to test whether there is
a next method.

If {\bf call-next-method} is used in methods that do not support it,
an error is signaled.

\label See Also:

``Method Selection and Combination''

``Standard Method Combination'' 

{\bf define-method-combination}

{\bf next-method-p}

\endcom


\begincom{cboundp}\ftype{Function}

\label Purpose:

The function {\bf cboundp} returns true if the given symbol names a class
in the given environment; otherwise it returns false.

\label Syntax:

\Defun {cboundp} {symbol {\opt} environment}

\label Arguments:

The optional {\it environment\/} argument is the same as the {\bf
\&environment} argument to macro expansion functions.  It is 
used to distinguish between compile-time and run-time environments.

If {\it environment\/} is not specified, the environment is the null
environment.

\label Values:

The function {\bf cboundp} returns true or false.

\label See Also:

{\bf symbol-class

cmakunbound}

\endcom



\begincom{change-class}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf change-class} changes the class of an
instance to a new class.  It destructively modifies and returns the
instance.

If in the old class there is any slot of the same name as a local
slot in the new class, the value of that slot is retained.  This
means that if the slot has a value, the value returned by {\bf
slot-value} after {\bf change-class} is invoked is {\bf eql} to the
value returned by {\bf slot-value} before {\bf change-class} is
invoked.  Similarly, if the slot was unbound, it remains
unbound.  The other slots are initialized as described in the
section ``Changing the Class of an Instance.''

\label Syntax:

\Defgen {change-class} {instance new-class}

\label Method Signatures:

\Defmeth {change-class} {({\it instance\/} standard-object) ({\it new-class\/} standard-class)}

\Defmeth {change-class} {({\it instance\/} t) ({\it new-class\/} symbol)}

\label Arguments:

The {\it instance\/} argument is a Lisp object.

The {\it new-class\/} argument is a class object or a symbol that names
a class. 

If the second of the above methods is selected, that method
invokes {\bf change-class} on {\it instance\/} and {\tt
(symbol-class {\it symbol\/})}.

\label Values:

The modified instance is returned.  The result of {\bf change-class}
is {\bf eq} to the {\it instance} argument.

\label Examples:

\screen!

(defclass position () ())

(defclass x-y-position (position)
    ((x :initform 0 :initarg :x)
     (y :initform 0 :initarg :y)))

(defclass rho-theta-position (position)
    ((rho :initform 0)
     (theta :initform 0)))

(defmethod class-changed ((old x-y-position) 
                          (new rho-theta-position))
  ;; Copy the position information from old to new to make new
  ;; be a rho-theta-position at the same position as old.
  (let ((x (slot-value old 'x))
        (y (slot-value old 'y)))
    (setf (slot-value new 'rho) (sqrt (+ (* x x) (* y y)))
          (slot-value new 'theta) (atan y x))))

;;; At this point an instance of the class x-y-position can be
;;; changed to be an instance of the class rho-theta-position using
;;; change-class:

(setq p1 (make-instance 'x-y-position :x 2 :y 0))

(change-class p1 'rho-theta-position)

;;; The result is that the instance bound to p1 is now an instance of
;;; the class rho-theta-position.   The method for class-changed
;;; performed the initializion of the rho and theta slots based
;;; on the value of the x and y slots, which were maintained by
;;; the old instance.

\endscreen!

\label Remarks:

After completing all other actions, {\bf change-class} invokes the generic
function {\bf class-changed}.  The generic function {\bf class-changed}
can be used to assign values to slots in the transformed instance.

The function {\bf change-class} has several semantic difficulties.
First, it performs a destructive operation that can be invoked within a
method on an instance that was used to select that method. When multiple
methods are involved because methods are being combined,
the methods currently executing or about to be executed
may no longer be applicable.  Second, some implementations might use compiler
optimizations of slot access, and when the class of an instance is
changed the assumptions the compiler made might be violated.
This implies that a programmer must not use {\bf
change-class} inside a method if any methods for that generic function 
access any slots, or else the results are undefined.

\vfill\eject
\label See Also:
 
``Changing the Class of an Instance''

{\bf class-changed}

\endcom




\begincom{class-changed}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf class-changed} initializes the newly-added
slots when an instance is transformed from the old version of a class
to the new version.

Each local slot of the new version of the class with no slot by the
same name in the old version of the class is initialized to the value
of the corresponding {\bf :initform} option of the new class or
remains unbound if the new version of the class does not specify
or inherit the {\bf :initform} option for that slot.  The {\bf :initform}
form is evalued in the lexical environment of its defining {\bf defclass}
form.

The generic function {\bf class-changed} is not intended to be called
by programmers.  Programmers are expected to write methods for it.
The function {\bf class-changed} is called only by the function {\bf
change-class}.  

Methods on {\bf class-changed} can be written to perform other
activities whenever an instance of one class is made to be an instance
of another class.

\label Syntax:

\Defgen {class-changed} {previous current}

\label Method Signatures:

\Defmeth {class-changed} {({\it previous\/} standard-object) ({\it current\/} standard-object)}

\label Arguments:
The arguments to {\bf class-changed} are computed by {\bf
change-class}.  When {\bf change-class} is invoked on an instance, a
copy of that instance is made; {\bf change-class} then destructively
alters the original instance.  The first argument to {\bf
class-changed}, {\it previous\/}, is that copy; it holds the old slot
values temporarily.  This argument has dynamic extent within {\bf
change-class}; if it is referenced in any way once {\bf class-changed}
returns, the results are undefined.  The second argument to {\bf
class-changed}, {\it current}, is the altered original instance.

The intended use of {\it previous\/} is to extract old slot values by using
{\bf slot-value} or {\bf with-slots} or by invoking an accessor generic
function, or to run other methods that were applicable to instances of
the original class.

\label Values:

The value returned by {\bf class-changed} is ignored by {\bf change-class}.

\vfill\eject
\label Examples:

See the example for the function {\bf change-class}.

\label Remarks:

Methods on {\bf class-changed} can be defined to initialize slots
differently from {\bf change-class}.  The default behavior of {\bf
change-class} is described in ``Changing the Class of an Instance.''

\label See Also:

``Changing the Class of an Instance''

{\bf change-class}

\endcom



\begincom{class-name, (setf class-name)}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf class-name} takes a class object and returns its
name.

The generic function {\bf (setf class-name)} takes a class object and sets
its name.

\label Syntax:

\Defgen {class-name} {class}

\Defgen {(setf class-name)} {new-value class}

\label Method Signatures:

\Defmeth {class-name} {({\it class\/} standard-class)}

\Defmeth {(setf class-name)} {({\it new-value\/} symbol) ({\it class\/} standard-class)}

\label Arguments:

The {\it class\/} argument is a class object.

The {\it new-value\/} argument is a symbol.

\label Values:

The name of the given class is returned; this result is a
symbol.

\label Remarks:

The name of an anonymous class is {\bf nil}.

If $S =${\tt (class-name $C$)} and $C =${\tt (symbol-class $S$)},
then $S$ is the proper name of $C$.  For further discussion, see
the section ``Classes.''

\label See Also:

{\bf symbol-class}

\endcom



\begincom{class-of}\ftype{Function}

\label Purpose:

The function {\bf class-of} returns the most specific class of which
the given object is an instance.

\label Syntax:

\Defun {class-of} {object}

\label Arguments:

The argument to {\bf class-of} may be any Common Lisp object.

\label Values:

The function {\bf class-of} returns the most specific class of which
the argument is an instance.

\label Remarks:

The Common Lisp types that have corresponding classes are listed in
Figure~1-1.

\endcom


\begincom{cmakunbound}\ftype{Function}

\label Purpose:

The function {\bf cmakunbound} causes the given symbol to no longer name
a class in the given environment.

\label Syntax:

\Defun {cmakunbound} {symbol {\opt} environment}

\label Arguments:

The optional {\it environment\/} argument is the same as the {\bf
\&environment} argument to macro expansion functions.  It is typically
used to distinguish between compile-time and run-time environments.

If {\it environment\/} is not specified, the environment is the null
environment.

\label Values:

The symbol is returned.

\label See Also:

{\bf symbol-class

cboundp}

\endcom



\begincom{defclass}\ftype{Macro}

\label Purpose:

The macro {\bf defclass} defines a new class.  It returns the new class
object as its result.

The syntax of {\bf defclass} provides options for specifying
initialization arguments for slots, for specifying default
initialization values for slots, and for requesting that methods on
specified generic functions be automatically generated for
reading and writing the values of slots.  No accessors or readers are
defined by default; their generation must be explicitly requested.

Defining a new class also causes a type of the same name to be
defined.  The predicate {\tt (typep {\it object class-name\/})} is
true if the class of the given object is {\it class-name\/} itself or
a subclass of the class {\it class-name}.  A class object can be used
as a type specifier.  Thus {\tt (typep {\it object class\/})} is true
if the class of the {\it object\/} is {\it class\/} itself or a
subclass of {\it class}.

\eject
\label Syntax:

\cboxfig{
\leftskip 2pc
\cleartabs\settabs\+\hskip\leftskip&\cr
\+&{\bf defclass} {\it class-name} \paren{\star{\curly{superclass-name}}}
\paren{\star{\curly{slot-spec}}}%
 $\lbrack\!\lbrack\downarrow\!\hbox{{\it class-option}}\,\rbrack\!\rbrack$\cr
\Vskip 1pc!
\+&{\it class-name\/}::$=$ {\it symbol\/}\cr
\Vskip 1pc!
\+&{\it superclass-name\/}::$=$ {\it symbol\/}\cr
\Vskip 1pc!
\+&\cleartabs{\it slot-spec\/}::$=$ &{\it slot-name\/} $\vert$ ({\it slot-name\/}  $\lbrack\!\lbrack\downarrow\!\hbox{{\it slot-option}}\,\rbrack\!\rbrack$)\cr
\Vskip 1pc!
\+&{\it slot-name\/}::$=$ {\it symbol\/}\cr
\Vskip 1pc!
\+&\cleartabs{\it slot-option\/}::$=$ &\star{\curly{{\tt :accessor} {\it generic-function-name\/}}} $\vert$ \cr
\+&&\star{\curly{{\tt :reader} {\it generic-function-name\/}}} $\vert$ \cr
\+&&\curly{{\tt :allocation} {\it allocation-type\/}} $\vert$ \cr
\+&&\star{\curly{{\tt :initarg} {\it initarg-name\/}}} $\vert$ \cr
\+&&\curly{{\tt :initform} {\it form\/}} $\vert$ \cr
\+&&\curly{{\tt :type} {\it type-specifier\/}} \cr
\Vskip 1pc!
\+&{\it generic-function-name\/}::$=$ {\it symbol\/}\cr
\Vskip 1pc!
\+&{\it initarg-name\/}::$=$ {\it symbol\/}\cr
\Vskip 1pc!
\+&{\it allocation-type\/}::$=$ {\tt :instance $\vert$ :class}\cr
\Vskip 1pc!
\+&\cleartabs{\it class-option\/}::$=$ &({\tt :default-initargs} {\it initarg-list\/}) $\vert$ \cr
\+&&({\tt :documentation} {\it string\/}) $\vert$ \cr
\+&&({\tt :metaclass} {\it class-name\/}) \cr
\Vskip 1pc!
\+&{\it initarg-list\/}::$=$ \star{\curly{\it initarg-name default-initial-value-form}} \cr
}
\caption{Syntax for defclass}
\endfig

\vfill\eject


\label Arguments:

The {\it class-name\/} argument is a non-{\bf nil} symbol.  It becomes
the proper name of the new class.  If a class with the same proper
name already exists and that class is an instance of {\bf
standard-class}, and the {\bf defclass} form for the definition of the
new class specifies a class of class {\bf standard-class}, the definition
of the existing class is replaced.

Each {\it superclass-name\/} argument is a non-{\bf nil} symbol that
specifies a direct superclass of the new class.  The new class will
inherit slots and methods from each of its direct superclasses, from
their direct superclasses, and so on.  See the section ``Inheritance''
for a discussion of how slots and methods are inherited.

Each {\it slot-spec\/} argument is the name of the slot or a list consisting
of the slot name followed by zero or more slot options.  The {\it slot-name\/}
argument is a symbol that can be used as a Common Lisp variable name.   
If there are any duplicate slot names, an error is signaled. 

The following slot options are available:

\beginlist

\item{\bull} 
The {\bf :accessor} slot option specifies that an unqualified method
is to be defined on the generic function named {\it
generic-function-name\/} to read the value of the given slot and that
an unqualified method is to be defined on the generic function named
{\tt (setf {\it generic-function-name\/})} to be used with {\bf setf}
to modify the value of the slot.  The {\it generic-function-name\/}
argument is a non-{\bf nil} symbol.  The {\bf :accessor} slot option
may be specified more than once for a given slot.

\item{\bull}
The {\bf :reader} slot option specifies that an unqualified method
is to be defined on the generic function named {\it
generic-function-name\/} to read the value of the given slot.  The
{\it generic-function-name\/} argument is a non-{\bf nil} symbol.  The
{\bf :reader} slot option may be specified more than once for a given slot.

\item{\bull} 
The {\bf :allocation} slot option is used to specify where storage is to be
allocated for the given slot.  Storage for a slot may be located in
each instance or in the class object itself.  The value of the {\it
allocation-type\/} argument can be one of the following keywords: {\bf
:instance} or {\bf :class}.  The {\bf :allocation} slot option may be
specified once at most for a given slot.  If the {\bf :allocation}
slot option is not specified, the effect is the same as specifying
{\bf :allocation :instance}.

\itemitem{--}
If {\it allocation-type\/} is {\bf :instance}, a local slot of the given name
is allocated in each instance of the class.  

\itemitem{--}
If {\it allocation-type\/} is {\bf :class}, a shared slot of the given
name is allocated in the class object created by this {\bf defclass}
form.  The value of the slot is shared by all instances of the class.
If a class $C\sub1$ defines such a shared slot, any subclass $C\sub2$ of
$C\sub1$ will share this single slot unless the {\bf defclass} form
for $C\sub2$ specifies a slot of the same name or there is a
superclass of $C\sub2$ that precedes $C\sub1$ in the class precedence
list of $C\sub2$ and that defines a slot of the same name.

\vfill\eject
\item{\bull} The {\bf :initform} slot option is used to provide a default
initial value form to be used in the initialization of the slot.  The
{\bf :initform} slot option may be specified once at most for a given slot.
This form is evaluated every time it is used.  The lexical environment
in which this form is evaluated is the lexical environment in which
{\bf defclass} was evaluated.  Note that the lexical environment
refers both to variables and to functions.  The dynamic environment is
the one in effect at the time the form is evaluated.  

No implementation is permitted to extend the syntax of {\bf defclass}
to allow {\tt ({\it slotname form\/})} as an abbreviation for {\tt
({\it slotname\/} :initform {\it form\/})}.

\item{\bull}
The {\bf :initarg} slot option declares an initialization argument
named {\it name\/} and specifies that this initialization argument
initializes the given slot.  If the initialization argument has a
value in the call to {\bf initialize-instance}, the value will be
stored into the given slot and the slot's {\bf :initform} argument, if
any, is not evaluated.  If none of the initialization arguments
specified for a given slot has a value, the slot is initialized
according to the {\bf :initform} argument, if specified.  The {\bf
:initarg} slot option can be specified more than once for a given
slot.  The {\it name\/} argument is any symbol.

\item{\bull}
The {\bf :type} slot option specifies that the contents of the slot
will always be of the specified data type.  It effectively declares
the result type of the reader generic function when applied to an
object of this class.  Attempting to store a value in a slot that does
not satisfy the type should signal an error.  The {\bf :type} slot
option may be specified once at most for a given slot.  The {\bf
:type} slot option is further discussed in the section ``Inheritance
of Slots and Slot Options.''


\endlist

Each class option is an option that refers to the class as a whole
or to all class slots.  The following class options are available:

\beginlist
\item{\bull}
The {\bf :default-initargs} class option is followed by a list of
alternating initialization argument names and default initial value
forms.  If any of these initialization arguments does not appear in
the initialization argument list supplied to {\bf make-instance}, the
corresponding default initial value form is evaluated, and the
initialization argument name and the form's value are added to the end
of the initialization argument list before the instance is created
(see the section ``Object Creation and Initialization'').  The default
initial value form is evaluated each time it is used.  The lexical
environment in which this form is evaluated is the lexical environment
in which the {\bf defclass} form was evaluated.  The dynamic
environment is the dynamic environment in which {\bf make-instance}
was called.  If an initialization argument name appears more than once
in a {\bf :default-initargs} class option, the leftmost is used.  The
{\bf :default-initargs} class option can be specified at most once.

\item{\bull} 
The {\bf :documentation} class option causes a documentation string to be
attached to the class name.  The documentation type for this string is
{\bf type}.  The form {\tt (documentation {\it class-name\/} 'type)}
may be used to retrieve the documentation string.  The {\bf
:documentation} class option may be specified once at most.

\vfill\eject
\item{\bull}
The {\bf :metaclass} class option is used to specify that instances of the
class being defined are to have a different metaclass than the default
provided by the system (the class {\bf standard-class}).  The {\it
class-name} argument is the name of the desired metaclass.  The {\bf
:metaclass} class option may be specified once at most.

\endlist

\label Values:

The new class object is returned as the result.

\label Remarks:

If a class with the same proper name already exists and that class is
an instance of {\bf standard-class}, and the {\bf defclass} form for
the definition of the new class specifies a class of class {\bf
standard-class}, the existing class is redefined and instances of it
(and its subclasses) are updated to the new definition at the time
that they are next accessed.  For details, see ``Redefining Classes.''

Note the following rules of {\bf defclass}:

\beginlist

\item{\bull}
It is not required that the superclasses of a class be defined before
the {\bf defclass} form for that class is evaluated.

\item{\bull}
All the superclasses of a class must be defined before 
an instance of the class can be made.

\item{\bull}
A class must be defined before it can be used as a parameter
specializer in a {\bf defmethod} form.

\endlist

The behavior of the \OS\ can be extended in cases where these rules are not
obeyed.

Some slot options are inherited by a class from its superclasses, and
some can be shadowed or altered by providing a local slot description.
No class options except {\bf :default-initargs} are inherited.  For a
detailed description of how slots and slot options are inherited, see
the section ``Inheritance of Slots and Slot Options.''

The options to {\bf defclass} can be extended.  It is required that
all implementations signal an error if they observe a class option or
a slot option that is not implemented locally.

It is valid to specify more than one accessor, reader, or
initialization argument for a slot.  No other slot option may appear
more than once in a single slot description, or else an error is
signaled.

If neither a reader nor an accessor is specified for a slot, the slot
can only be accessed by the function {\bf slot-value}.

\vfill\eject
\label See Also:

{\bf slot-value

make-instance

initialize-instance}

``Classes''

``Inheritance''

``Redefining Classes''

``Determining the Class Precedence List''

``Object Creation and Initialization''
\endcom



\begincom{defgeneric}\ftype{Macro}

\label Purpose:

The macro {\bf defgeneric} is used to define a generic function or to
specify options and declarations that pertain to a generic function as
a whole.

%The {\it function-specifier\/} argument names the generic function.
If {\tt (fboundp {\it function-specifier\/})} is {\bf nil}, a new
generic function is created.  If {\tt (symbol-function {\it
function-specifier\/})} is a generic function, that generic function
is modified.  If {\it function-specifier\/} names a non-generic
function, a macro, or a special form, an error is signaled.

Each {\it method-description\/} defines a method on the generic function.
The lambda-list of each method must be congruent with the lambda-list
specified by the {\it lambda-list\/} option.  If this condition
does not hold, an error is signaled.  See the section ``Congruent
Lambda-lists for All Methods of a Generic Function'' for a definition
of congruence in this context.

The macro {\bf defgeneric} returns the generic function object 
as its result.

\label Syntax:

\Defmac {defgeneric} {function-specifier lambda-list\/ $\lbrack\!\lbrack\downarrow\!$ option $\vert$ \star{method-description} $\rbrack\!\rbrack$}

{\it function-specifier\/}::$=$ \curly{{\it symbol\/} $\vert$ {\tt (setf {\it symbol\/})}}
\Vskip 1pc!
\settabs\+\hskip\leftskip&\cr
\+&{\it lambda-list\/}::$=$ (&\star{\curly{var}}  \cr
\+&&\ttbrac{{\opt} \star{\curly{var $\vert$ {\rm (}var\/{\rm )}}}}  \cr
\+&&\ttbrac{{\tt\&rest} {\it var\/}} \cr
\+&&{\tt [}{\key{}}&\star{\curly{var  $\vert$
{\rm (}\curly{var $\vert$ {\rm (}keyword var\/{\rm )}}{\rm )}}}\cr
\+&&&\brac{\tt\&allow-other-keys} {\tt ]} {\rm )} \cr
\Vskip 1pc!
{\it option\/}::$=$ {\tt\vtop{\hbox{(:argument-precedence-order \plus{\curly{parameter-name}}) $\vert$}
\hbox{(declare \plus{\curly{declaration\/}}) $\vert$}
\hbox{(:documentation {\it string\/}) $\vert$}
\hbox{(:method-combination {\it symbol\/} \star{\curly{arg\/}}) $\vert$}
\hbox{(:generic-function-class {\it class-name\/}) $\vert$}
\hbox{(:method-class {\it class-name\/})}}}
\Vskip 1pc!
{\it method-description\/}::$=$ {\tt (:method {\it method-specifier\/})}
\Vskip 1pc!
{\it method-specifier\/}::$=$ \vtop{\hbox{\it \star{\curly{method-qualifier\/}} specialized-lambda-list\/}
\hbox{\star{\curly{declaration\/ $\vert$ documentation\/}} \star{\curly{form\/}}}}
\Vskip1pc!\null
{\it method-qualifier\/}::$=$ {\it non-nil-atom}
\Vskip1pc!\null
\settabs\+\hskip\leftskip&\cr
\+&{\it specialized-lambda-list\/}::$=$ (&\star{\curly{var  $\vert$ {\rm (}var parameter-specializer-name\/{\rm )}}}  \cr
\+&&\ttbrac{{\opt} \star{\curly{var $\vert$ {\rm (}var \ttbrac{initform {\brac{supplied-p-parameter}} }{\rm )}}}}  \cr
\+&&\ttbrac{{\tt\&rest} {\it var\/}} \cr
\+&&{\tt [}{\key{}}&\star{\curly{var  $\vert$
{\rm (}\curly{var $\vert$ {\rm (}keyword var{\rm )}}
\ttbrac{initform \brac{supplied-p-parameter} }{\rm )}}}\cr
\+&&&\brac{\tt\&allow-other-keys} {\tt ]} \cr
\+&&\ttbrac{{\tt\&aux} \star{\curly{var $\vert$ {\rm (}var \brac{initform} {\rm )}}}} {\rm )} \cr
\settabs\+\hskip\leftskip&\cr
\Vskip1pc!\null
\+&{\it parameter-specializer-name\/}::$=$ {\it symbol} $\vert$ {\rm (}{\tt eql} {\it eql-specializer-form\/}{\rm )}\cr
\Vskip 1pc!

\label Arguments:

The {\it function-specifier} argument is a non-{\bf nil} symbol or a
list of the form {\tt (setf {\it symbol\/})}.

The {\it lambda-list\/} argument is an ordinary function lambda-list
with these exceptions:

\beginlist
\item{\bull}
The use of {\bf \&aux} is not allowed. 

\item{\bull}
Optional and keyword arguments may not have default initial value forms
nor use supplied-p parameters.
The generic function passes to the method all the argument values passed to
it, and only those; default values are not supported.
Note that optional and keyword arguments in method definitions, however,
can have default initial value forms and can use supplied-p parameters. 
\endlist

The following options are provided.  A given option may occur only once,
or else an error is signaled.

\beginlist
 
\item{\bull} 
The {\bf :argument-precedence-order} option is used to specify the
order in which the required arguments in a call to the generic
function are tested for specificity when selecting a particular
method.  Each required argument, as specified in the {\it lambda-list\/}
argument, must be included exactly once as a {\it
parameter-name} so that the full and unambiguous precedence order is
supplied.  If this condition is not met, an error is signaled.

\item{\bull}
The {\bf declare} option is used to specify declarations that pertain
to the generic function.  The following standard Common Lisp
declaration is allowed:

\itemitem{--}
An {\bf optimize} declaration specifies whether method selection
should be optimized for speed or space, but it has no effect on
methods.  To control how a method is optimized, an {\bf optimize}
declaration must be placed directly in the {\bf defmethod} form or
method description.  The optimization qualities {\bf speed} and {\bf
space} are the only qualities this standard requires, but an
implementation can extend the \CLOS\ to recognize other qualities.  A
simple implementation that has only one method selection technique and
ignores the {\bf optimize} declaration is valid.

\item{}
The {\bf special}, {\bf ftype}, {\bf function}, {\bf inline}, {\bf
notinline}, and {\bf declaration} declarations are not permitted.
Individual implementations can extend the {\bf declare} option to
support additional declarations.  If an implementation notices a
declaration that it does not support and that has not been proclaimed
as a nonstandard declaration name in a {\bf proclain} proclamation, it
should issue a warning.

\item{\bull} 
The {\bf :documentation} argument associates a documentation string
with the generic function.  The documentation type for this string is
{\bf function}.  The form {\tt (documentation {\it
function-specifier\/} 'function)} may be used to retrieve this
string.

\item{\bull} 
The {\bf :generic-function-class} option may be used to specify that
the generic function is to have a different class than the default
provided by the system (the class {\bf standard-generic-function}).
The {\it class-name\/} argument is the name of a class that can be the
class of a generic function.  If {\it function-specifier\/} specifies
an existing generic function that has a different value for the {\bf
:generic-function-class} argument and the new generic function class
is compatible with the old, {\bf change-class} is called to change the
class of the generic function, otherwise an error is signaled.

\item{\bull} 
The {\bf :method-class} option is used to specify that all methods on
this generic function are to have a different class than the default
provided by the system (the class {\bf standard-method}).  The {\it
class-name\/} argument is the name of a class that is capable of being
the class of a method.

\item{\bull} 
The {\bf :method-combination} option is followed by a symbol that
names a type of method combination.  The arguments (if any) that
follow that symbol depend on the type of method combination.  Note
that the standard method combination type does not support any
arguments.  However, all types of method combination defined by the
short form of {\bf define-method-combination} accept an optional
argument named {\it order\/}, defaulting to {\bf
:most-specific-first}, where a value of {\bf :most-specific-last} reverses
the order of the primary methods, without affecting the order of the
auxiliary methods.

\endlist

The {\it method-description\/} arguments define the methods that are
to be added to the generic function.  The {\it method-qualifier} and
{\it specialized-lambda-list} arguments in a method description are
the same as for {\bf defmethod}.

The {\it form\/} arguments specify the method body.
The body of the method is enclosed in an implicit block.  If
{\it function-specifier\/} is a symbol, this block bears the same name as the
generic function.  If {\it function-specifier\/} is a list of the form {\tt
(setf {\it symbol\/})}, the name of the block is {\it symbol}.  The {\bf
return-from} construct may be used to cause an exit from the method as
well as the block.

\label Values:

The generic function object is returned as the result. 

\label Remarks:

The macro {\bf defgeneric} is implemented in terms of {\bf
ensure-generic-function} followed by zero or more invocations of {\bf
defmethod}.

If a generic function of the same name already exists, and a method
with the same parameter specializers and the same qualifiers as a
method specified by a method description in the {\bf defgeneric} form
already exists on that generic function, the existing method is
replaced.  If the {\bf defgeneric} form specifies no method
descriptions, all methods on the existing generic function are
retained.

If no method descriptions are specified and a generic function of the same
name does not already exist, a generic function with no methods is created.

The {\it lambda-list\/} argument of {\bf
defgeneric} specifies the shape of lambda-lists for the methods on
this generic function.  All methods on the generic function must have
lambda-lists that are congruent with this shape.  If a {\bf
defgeneric} form is evaluated and some methods for that generic
function have lambda-lists that are not congruent with that given in
the {\bf defgeneric} form, an error is signaled.  For further details
on method congruence, see ``Congruent Lambda-lists for all Methods of
a Generic Function''

Implementations can extend {\bf defgeneric} to include other options.
It is required that an implementation signal an error if
it observes an option that is not implemented locally.

\label See Also:

``Congruent Lambda-lists for All Methods of a Generic Function'' 

{\bf defmethod

ensure-generic-function

generic-function}

\endcom




\begincom{define-method-combination}\ftype{Macro}

\label Purpose:

The macro {\bf define-method-combination} is used to define new types
of method combination.

There are two forms of {\bf define-method-combination}.  The short
form is a simple facility for the cases that are expected
to be most commonly needed.  The long form is more powerful but more
verbose.  It resembles {\bf defmacro} in that the body is an
expression, usually using backquote, that computes a Lisp form.  Thus
arbitrary control structures can be implemented.  The long form also
allows arbitrary processing of method qualifiers.

\label Syntax:

\Defmac {define-method-combination} {name \star{\curly{short-form-option}}}
\Vskip 1pc!
{\it short-form-option\/}::$=$ {\tt\vtop{\hbox{:documentation {\it string\/} $\vert$}
\hbox{:identity-with-one-argument {\it boolean\/} $\vert$}
\hbox{:operator {\it operator\/} $\vert$}}}
\Vskip1pc!

\Defmac {define-method-combination} {\vtop{\hbox{name lambda-list }
\hbox{\paren{\star{\curly{method-group-specifier}}} }
\hbox{\star{\curly {declaration $\vert$ doc-string}}}
\hbox{\star{\curly{form}}}}}
\vbox
{\halign {\hskip \leftskip#\hfil&#\hfil\cr
{\it method-group-specifier\/}::$=$ (&{\it variable\/}
\curly{\plus{\curly{qualifier-pattern}} $\vert$ predicate}\cr
&\star{\curly{long-form-option}})\cr}}
\Vskip 1pc!
{\it long-form-option\/}::$=$ {\tt\vtop{\hbox{:description {\it format-string\/} $\vert$}
\hbox{:order {\it order\/} $\vert$}
\hbox{:required {\it boolean\/}}}}
\Vskip1pc!\null

\label Arguments:

In both the short and long forms, {\it name\/} is a symbol.  By convention,
non-keyword, non-{\bf nil} symbols are usually used.

\vfill\eject
\label Arguments of the Short Form:

The short form syntax of {\bf define-method-combination} is recognized
when the second subform is a non-{\bf nil} symbol or is not present.
When the short form is used, {\it name\/} is defined as a type of
method combination that produces a Lisp form {\tt ({\it operator
method-call method-call $\ldots$ })}.  The {\it operator\/} is a symbol
that can be the name of a function, macro, or special form.  The
{\it operator\/} can be specified by a keyword option; it defaults to {\it
name}.

Keyword options for the short form are the following:

\beginlist

\item{\bull}
The {\bf :documentation} option is used to document the method-combination type.

\item{\bull}
The {\bf :identity-with-one-argument} option enables an optimization
when {\it boolean\/} is true (the default is false).  If there is
exactly one applicable method and it is a primary method, that method
serves as the effective method and {\it operator\/} is not called.
This optimization avoids the need to create a new effective method and
avoids the overhead of a function call.  This option is designed to be
used with operators such as {\bf progn}, {\bf and}, {\bf $+$}, and
{\bf max}.

\item{\bull}
The {\bf :operator} option specifies the name of the operator.  The
{\it operator\/} argument is a symbol that can be the name of a
function, macro, or special form.  By convention, {\it name\/} and
{\it operator\/} are often the same symbol.  This is the default,
but it is not required.

\endlist

None of the subforms is evaluated.

These types of method combination require exactly one qualifier per
method.  An error is signaled if there are applicable methods with no
qualifiers, or with qualifiers that are not supported by the method
combination type. 

A method combination procedure defined in this way recognizes two
roles for methods.  A method whose one qualifier is the symbol naming
this type of method combination is defined to be a primary method.  At
least one primary method must be applicable or an error is signaled.
A method with {\bf :around} as its one qualifier is an auxiliary
method that behaves the same as a {\bf :around} method in standard
method combination.  The function {\bf call-next-method} can only be
used in {\bf :around} methods; it cannot be used in primary methods
defined by the short form of the {\bf define-method-combination} macro.

A method combination procedure defined in this way accepts an optional
argument named {\it order}, which defaults to {\bf
:most-specific-first}.  A value of {\bf :most-specific-last} reverses
the order of the primary methods without affecting the order of the
auxiliary methods.

A large fraction of the types of method combination needed by 
most programmers can be implemented with this short form, which is 
provided for convenience.   The short form automatically includes 
error checking and support for {\bf :around} methods
and avoids the need for the use of the backquote and comma.

For a discussion of built-in method combination types, see the section
``Built-in Method Combination Types.''

\eject
\label Arguments of the Long Form:

The long form syntax of {\bf define-method-combination} is recognized 
when the second subform is a list.  

The {\it lambda-list\/} argument is an ordinary lambda-list.  It
receives any arguments provided after the name of the method
combination type in the {\bf :method-combination} option to {\bf
defgeneric}.

A list of method-group specifiers follows.  Each specifier selects a subset
of the applicable methods to play a particular role, either by matching
their qualifiers against some patterns or by testing their qualifiers with
a predicate.   These method-group specifiers define all method qualifiers
that can be used with this type of method combination.  If an applicable 
method does not fall into any method-group, the system signals the error
that the method is invalid for the kind of method combination in use.

Each method-group specifier names a variable.  During the execution of
the forms in the body of {\bf define-method-combination}, this
variable is bound to a list of the methods in the method-group.  The
methods in this list occur in most-specific-first order.

A qualifier pattern is a list or the symbol {\bf *}.  A method matches
a qualifier pattern if the method's list of qualifiers is {\bf equal}
to the qualifier pattern (except that the symbol {\bf *} in a qualifier
pattern matches anything).  Thus, a qualifier pattern can be one of the
following: the empty list {\bf ()}, which matches unqualified methods;
the symbol {\bf *}, which matches all methods; a true list, which
matches methods with the same number of qualifiers as the length of
the list when each qualifier matches the corresponding list element;
or a dotted list that ends in the symbol {\bf *} (the {\bf *} matches
any number of additional qualifiers).


Each applicable method is tested against the qualifier patterns and
predicates in left-to-right order.  As soon as a qualifier pattern matches
or a predicate returns true, the method becomes a member of the
corresponding method-group and no further tests are made.  Thus, if a method
could be a member of more than one method-group, it joins only the first
such group.  If a method-group has more than one qualifier pattern, a
method need only satisfy one of the qualifier patterns to be a member of
the group.

The name of a predicate function can appear instead of qualifier
patterns in a method-group specifier.  The predicate is called for
each method that has not been assigned to an earlier method-group; it
is called with one argument, the method's qualifier list.  The
predicate should return true if the method is to be a member of the
method-group.  A predicate can be distinguished from a qualifier pattern
because it is a symbol other than {\bf nil} or {\bf *}.

Method-group specifiers can have keyword options following the
qualifier patterns or predicate.  Keyword options can be distinguished from
additional qualifier patterns because they are neither lists nor the symbol
{\bf *}.  The keyword options are as follows:

\beginlist

\item{\bull}
The {\bf :description} option is used to provide a description of the
role of methods in the method-group.  Programming environment tools
use {\tt (apply \#'format stream {\it format-string\/}
(method-qualifiers {\it method\/}))} to print this description, which
is expected to be concise, that is, one or two words.  This keyword
option allows the description of a method qualifier to be defined in
the same module that defines the meaning of the method
qualifier.  In most cases, {\it format-string\/} will not contain any
format directives, but they are available for generality.  If {\bf
:description} is not specified, a default description is generated
based on the variable name and the qualifier patterns and on whether
this method-group includes the unqualified methods.  The argument {\it
format-string\/} is not evaluated.

\item{\bull}
The {\bf :order} option specifies the order of methods.  The {\it
order\/} argument is a form that evaluates to {\bf
:most-specific-first} or {\bf :most-specific-last}.  If it evaluates
to any other value, an error is signaled.  This keyword option is a
convenience and does not add any expressive power.
If {\bf :order} is not specified, it defaults to {\bf :most-specific-first}.

\item{\bull}
The {\bf :required} option specifies whether at least one method in
this method-group is required.  If the {\it boolean\/} argument is
non-{\bf nil} and the method-group is empty (that is, no applicable
methods match the qualifier patterns or satisfy the predicate), an
error is signaled.  This keyword option is a convenience and does not
add any expressive power.  If {\bf :required} is not specified,
it defaults to {\bf nil}.  The {\it boolean\/} argument is not
evaluated.

\endlist

The use of method-group specifiers provides a convenient syntax to
select methods, to divide them among the possible roles, and to perform the
necessary error checking.  It is possible to perform further filtering
of methods in the body forms by using normal list-processing operations
and the functions {\bf method-qualifiers} and {\bf
invalid-method-error}.  It is permissible to use {\bf setq} on the
variables named in the method-group specifiers and to bind additional
variables.  It is also possible to bypass the method-group specifier
mechanism and do everything in the body forms.  This is accomplished
by writing a single method group with {\bf *} as its only qualifier
pattern; the variable is then bound to a list of all of the applicable
methods, in most specific first order.

The body {\it forms\/} compute and return the Lisp form that specifies how
the methods are combined, that is, the effective method.
The body of {\bf define-method-combination} resembles the body of
{\bf defmacro} and uses backquote in a similar way.
The function {\bf make-method-call} is also used in constructing the
Lisp form; it hides the implementation-dependent details of how
methods are called.  Programmers always use {\bf make-method-call} to
translate from the lists of method objects produced by the method-group
specifiers to Lisp forms that invoke those methods.
 
Erroneous conditions detected by the body should be reported with
{\bf method-combination-error} or {\bf invalid-method-error}; these functions
add any necessary contextual information to the error message and will
signal the appropriate error.

The body {\it forms\/} are evaluated inside of the bindings created by the
lambda-list and method-group specifiers.  Declarations at the head of
the body are positioned directly inside of bindings created by the
lambda-list and outside of the bindings of the method-group variables. 
Thus method-group variables cannot be declared.

Within the body {\it forms\/}, the lexical variable {\bf generic-function}
is bound to the generic-function object.

If a {\it doc-string\/} argument is present, it documents the
method-combination type.

\vfill\eject
The functions {\bf make-method-call}, {\bf method-combination-error},
and {\bf invalid-method-error} can be called from the body {\it
forms\/} or from functions called by the body {\it forms\/}.  The
actions of these three functions can depend on dynamic variables
automatically bound before the generic function {\bf
compute-effective-method} is called.  These variables might contain
the parameter list of the effective method or other
implementation-dependent information.

Note that two methods with identical specializers, but different
qualifiers, are not ordered by the algorithm described in Step 2 of
the method selection and combination process described in the section
``Method Selection and Combination.''  Normally the two methods play
different roles in the effective method because they have different
qualifiers, and no matter how they are ordered in the result of Step
2, the effective method is the same.  If the two methods play the same
role and their order matters, an error is signaled.  This happens as
part of the qualifier pattern matching in {\bf
define-method-combination}.

\label Values:

The value returned by the {\bf define-method-combination} macro is the name of
the new type of method combination.

\label Examples:

Most examples of the long form of {\bf define-method-combination} also
illustrate the use of the related functions that are provided as part
of the declarative method combination facility.

\screen!
;;; Examples of the short form of define-method-combination

(define-method-combination and :identity-with-one-argument t) 

(defmethod func and ((x class1) y) ...)

;;; The equivalent of this example in the long form is:

(define-method-combination and 
        (&optional (order ':most-specific-first))
        ((around (:around))
         (primary (and) :order order :required t))
  (make-method-call `(,@around
                      ,(make-method-call primary
                                         :operator 'and
                                         :identity-with-one-argument t))
                    :operator :call-next-method))

;;; Examples of the long form of define-method-combination

;The default method-combination technique
(define-method-combination standard ()
        ((around (:around))
         (before (:before))
         (primary () :required t)
         (after (:after)))
  (make-method-call `(,@around
                      (multiple-value-prog1
                        (progn
                          ,(make-method-call before)
                          ,(make-method-call primary
                                             :operator :call-next-method))
                        ,(make-method-call (reverse after))))
                    :operator :call-next-method))

;A simple way to try several methods until one returns non-nil
(define-method-combination and ()
        ((methods (and)))
  (make-method-call methods :operator 'and))

;A more complete version of the preceding
(define-method-combination and 
        (&optional (order ':most-specific-first))
        ((around (:around))
         (primary (and)))
  ;; Process the order argument
  (case order
    (:most-specific-first)
    (:most-specific-last (setq primary (reverse primary)))
    (otherwise (method-combination-error "~S is an invalid order.~@
    :most-specific-first and :most-specific-last are the possible values."
                                         order)))
  ;; Must have a primary method
  (unless primary
    (method-combination-error "A primary method is required."))
  (make-method-call `(,@around
                      ,(make-method-call primary
                                         :operator 'and
                                         :identity-with-one-argument t))
                    :operator :call-next-method))

;The same thing, using the :order and :required keyword options
(define-method-combination and 
        (&optional (order ':most-specific-first))
        ((around (:around))
         (primary (and) :order order :required t))
  (make-method-call `(,@around
                      ,(make-method-call primary
                                         :operator 'and
                                         :identity-with-one-argument t))
                    :operator :call-next-method))

;This short-form call is behaviorally identical to the preceding
(define-method-combination and :identity-with-one-argument t)
 
;Order methods by positive integer qualifiers
;:around methods are disallowed to keep the example small
(define-method-combination example-method-combination ()
        ((methods positive-integer-qualifier-p))
  (make-method-call (stable-sort methods #'<
                       :key #'(lambda (method)
                                (first (method-qualifiers method))))))

(defun positive-integer-qualifier-p (method-qualifiers)
  (and (= (list-length method-qualifiers) 1)
       (typep (first method-qualifiers) '(integer 0 *))))

\endscreen!


\label Remarks:

The {\bf :method-combination} option of {\bf defgeneric} is used to
specify that a generic function should use a particular method
combination type.  The argument to the {\bf :method-combination}
option is the name of a method combination type.

Individual implementations might support other keyword options.
Therefore, it is required that all implementations signal an error if
they observe a keyword option that is not implemented locally.

 
\label See Also: 

{\bf make-method-call}

{\bf method-qualifiers}

{\bf method-combination-error}

{\bf invalid-method-error} 

{\bf defgeneric}

\endcom



\begincom{defmethod}\ftype{Macro}

\label Purpose:

The macro {\bf defmethod} defines a method on a generic function.  

If a generic function is currently named by {\it function-specifier\/},  where
{\it function-specifier\/} is a symbol or a list of the form {\tt (setf {\it symbol\/})},
the lambda-list of the method must be congruent with the lambda-list
of the generic function.  If this condition does not hold, an error is
signaled.  See the section ``Congruent Lambda-lists for All Methods of
a Generic Function'' for a definition of congruence in this context.

If {\tt (fboundp {\it function-specifier\/})} is {\bf nil}, a generic
function is created with default values for the argument precedence
order (each argument is more specific than the arguments to its right
in the argument list), for the generic function class (the class {\bf
standard-generic-function}), for the method class (the class {\bf
standard-method}), and for the method combination type (the standard
method combination type).  The lambda-list of the generic function is
congruent with the lambda-list of the method being defined.  If {\it
function-specifier\/} names a non-generic function, a macro, or a
special form, an error is signaled.

\label Syntax:

\Defmac {defmethod} 
{\vtop{\hbox{function-specifier \star{\curly{method-qualifier\/}} specialized-lambda-list}
\hbox{\star{\curly{declaration $\vert$ documentation}} \star\form}}}
\Vskip1pc!\null
{\it function-specifier\/}::$=$ \curly{{\it symbol\/} $\vert$ {\tt (setf {\it symbol\/})}}
\Vskip1pc!\null
{\it method-qualifier\/}::$=$ {\it non-nil-atom}
\Vskip1pc!\null
\settabs\+\hskip\leftskip&\cr
\+&{\it specialized-lambda-list\/}::$=$ (&\star{\curly{var  $\vert$ {\rm (}var parameter-specializer-name\/{\rm )}}}  \cr
\+&&\ttbrac{{\opt} \star{\curly{var $\vert$ {\rm (}var \ttbrac{initform {\brac{supplied-p-parameter}} }{\rm )}}}}  \cr
\+&&\ttbrac{{\tt\&rest} {\it var\/}} \cr
\+&&{\tt [}{\key{}}&\star{\curly{var  $\vert$
{\rm (}\curly{var $\vert$ {\rm (}keyword var{\rm )}}
\ttbrac{initform \brac{supplied-p-parameter} }{\rm )}}}\cr
\+&&&\brac{\tt\&allow-other-keys} {\tt ]} \cr
\+&&\ttbrac{{\tt\&aux} \star{\curly{var $\vert$ {\rm (}var \brac{initform} {\rm )}}}} {\rm )} \cr
\Vskip1pc!\null
\+&{\it parameter-specializer-name\/}::$=$ {\it symbol} $\vert$ {\rm (}{\tt eql} {\it eql-specializer-form\/}{\rm )}\cr
\Vskip 1pc!

\vfill\eject
\label Arguments:

The {\it function-specifier\/} argument is a non-{\bf nil} symbol or a
list of the form {\tt (setf {\it symbol\/})}.  It names the generic
function on which the method is defined.

Each {\it method-qualifier\/} argument is an object that is used by
method combination to identify the given method.  A method qualifier
is a non-{\bf nil} atom.  The method combination type may further
restrict what a method qualifier may be.  The standard method
combination type allows for unqualified methods or methods whose sole
qualifier is one of the following keywords: {\bf :before}, {\bf
:after}, or {\bf :around}.

The {\it specialized-lambda-list\/} argument is like an ordinary
function lambda-list except that the names of required parameters can
be replaced by specialized parameters.  A specialized parameter is a
list of the form {\tt ({\it variable-name
parameter-specializer-name\/})}.  Only required parameters may be
specialized.  A parameter specializer name is a symbol that names a
class or {\tt (eql {\it eql-specializer-form\/})}.  The parameter
specializer name {\tt (eql {\it eql-specializer-form\/})} indicates
that the corresponding argument must be {\bf eql} to the object that
is the value of {\it eql-specializer-form\/} for the method to be
applicable.  If no parameter specializer name is specified for a given
required parameter, the parameter specializer defaults to the class
named {\bf t}.  See the section ``Introduction to Methods'' for
further discussion.

The {\it form\/} arguments specify the method body.
The body of the method is enclosed in an implicit block.  If
{\it function-specifier\/} is a symbol, this block bears the same name as the
generic function.  If {\it function-specifier\/} is a list of the form {\tt
(setf {\it symbol\/})}, the name of the block is {\it symbol}.  The {\bf
return-from} construct may be used to cause an exit from the method as
well as the block.

\label Values:

The result of {\bf defmethod} is the method object.

\label Remarks:

The class of the method object that is created is that given by the 
method class option of the generic function on which the method is defined.

If a method already exists on the given generic function with 
parameter specializers and qualifiers that are the same ({\bf equal})
as those of the method being defined, {\bf defmethod} replaces
the existing method with the one now being defined. 

The parameter specializers are derived from the parameter specializer
names as described in the section ``Introduction to Methods.''

The expansion of the {\bf defmethod} macro ``refers to'' each
specialized parameter (see the description of {\bf ignore} in {\it
Common Lisp: The Language}, p. 160).  This includes parameters that
have an explicit parameter specializer name of {\bf t}.  This means
that a compiler warning does not occur if the body of the method does
not refer to a specialized parameter.  Note that a parameter that
specializes on {\bf t} is not synonymous with an unspecialized
parameter in this context.

\vfill\eject
\label See Also:

{\bf add-method}

``Introduction to Methods''

``Congruent Lambda-lists for all Methods of a Generic Function'' 

\endcom


\begincom{describe}\ftype{Standard Generic Function}

\label Purpose:

The Common Lisp function {\bf describe} is replaced by a generic
function.  The generic function {\bf describe} prints information
about a given object on the standard output.

Each implementation is required to provide a default method, that is,
a method on the class {\bf t}, and a method on the class {\bf
standard-object}.  Implementations are free to add methods for
specific classes.  Users can write methods for {\bf describe} for
their own classes if they do not wish to inherit an
implementation-supplied method.  These methods must conform to the
definition of {\bf describe} as specified in {\it Common Lisp: The
Language}.

\label Syntax:

\Defgen {describe} {object}

\label Method Signatures:

\Defmeth {describe} {({\it object\/} t)}

\Defmeth {describe} {({\it object\/} standard-object)}


\label Arguments:

The {\it object\/} argument may be any Common Lisp object. 

\label Values:

The generic function {\bf describe} returns no values.

\endcom
 

\begincom{documentation, (setf documentation)}\ftype{Standard Generic Function}

\label Purpose:

The Common Lisp function {\bf documentation} is replaced by a generic
function.  The generic function {\bf documentation} returns the
documentation string associated with the given object if it is
available; otherwise it returns {\bf nil}.

The generic function {\bf (setf documentation)} is used to update the
documentation.

\label Syntax:

\Defgen documentation {x {\opt} doc-type}

\Defgen {(setf documentation)} {new-value x {\opt} doc-type}

\label Method Signatures:

\Defmeth {documentation} {({\it method\/} standard-method) \&optional {\it doc-type}}

\Defmeth {(setf documentation)} {\vtop{\hbox{({\it new-value\/} string) ({\it method\/} standard-method)}
\hbox{\&optional {\it doc-type}}}}

\Defmeth {documentation} {\vtop{\hbox{({\it generic-function\/} standard-generic-function)} \hbox{\&optional {\it doc-type}}}}

\Defmeth {(setf documentation)} {\vtop{\hbox{({\it new-value\/} string)}
\hbox{({\it generic-function\/} standard-generic-function)} 
\hbox{\&optional {\it doc-type}}}}

\Defmeth {documentation} {({\it class\/} standard-class) \&optional {\it doc-type}}

\Defmeth {(setf documentation)} {\vtop{\hbox{({\it new-value\/} string) ({\it class\/} standard-class)}
\hbox{\&optional {\it doc-type}}}}

\Defmeth {documentation} {({\it symbol\/} symbol) \&optional {\it doc-type}}

\Defmeth {(setf documentation)} {\vtop{\hbox{({\it new-value\/} string) ({\it symbol\/} symbol)}
\hbox{\&optional {\it doc-type}}}}

\Defmeth {documentation} {({\it list\/} list) \&optional {\it doc-type}}

\Defmeth {(setf documentation)} {\vtop{\hbox{({\it new-value\/} string) ({\it list\/} list)}
\hbox{\&optional {\it doc-type}}}}

\label Arguments:

The first argument of {\bf documentation} is either a symbol, a
function specifier list of the form {\tt (setf {\it symbol\/})}, a
method object, a class object, or a generic function object.

\beginlist

\item{\bull}
If the first argument is a method object, a class object, or a generic
function object, the second argument must not be supplied, or an error
is signaled.

\item{\bull} 
If the first argument is a symbol or a list of the form
{\tt (setf {\it symbol\/})}, the second argument must be
supplied and must be one of the following symbols, or an error is
signaled: {\bf variable}, {\bf function}, {\bf structure},
{\bf type}, {\bf setf}, and {\bf method-combination}.

\itemitem{--}
The forms {\tt (documentation {\it symbol\/} 'function)} and
{\tt (documentation '(setf {\it symbol\/}) 'function)} return the
documentation string of the function, generic function, special form, or
macro named by the symbol or list.

\itemitem{--}
The form {\tt (documentation {\it symbol\/} 'variable)} returns the
documentation string of the special variable or constant named by the
symbol.

\itemitem{--}
The form {\tt (documentation {\it symbol\/} 'structure)} returns the
documentation string of the {\bf defstruct} structure named by the
symbol.

\itemitem{--}
The form {\tt (documentation {\it symbol\/} 'type)} returns the documentation
string of the class object named by the symbol, if there is such a
class.   If there is no such class, it returns the documentation string
of the type specifier named by the symbol. 

\itemitem{--}
The form {\tt (documentation {\it symbol\/} 'setf)} returns the documentation
string of the {\bf defsetf} or {\bf define-setf-method} definition
associated with the symbol.

\itemitem{--}
The form {\tt (documentation {\it symbol\/} 'method-combination)} returns the
documentation string of the method combination type named by the
symbol.  

\endlist

The first argument of {\bf (setf documentation)} is a string; the
remaining arguments are the same as the arguments of {\bf documentation}.
  
\label Values:

The documentation string associated with the given object is returned
unless none is available, in which case {\bf documentation} returns
{\bf nil}.

\endcom



\begincom{ensure-generic-function}\ftype{Function}

\label Purpose:

The function {\bf ensure-generic-function} is used to define a generic function
with no methods or to specify or modify options and declarations that pertain
to a generic function as a whole.

If {\tt (fboundp {\it function-specifier\/})} is {\bf nil}, a new
generic function is created.  If {\tt (symbol-function {\it
function-specifier\/})} is a non-generic function, a macro, or a
special form, an error is signaled.

If {\it function-specifier\/} specifies a generic function that has a
different value for any of the following arguments, the generic
function is modified to have the new value: {\bf
:argument-precedence-order}, {\bf :declare}, {\bf :documentation},
{\bf :method-combination}.

If {\it function-specifier\/} specifies a generic function that has a
different value for the {\bf :lambda-list} argument, but the two
values are congruent or there are no methods, the value is changed,
otherwise an error is signaled.

If {\it function-specifier\/} specifies a generic function that has a different value for
the {\bf :generic-function-class} argument and the new generic function class
is compatible with the old, {\bf change-class} is called to change the class of
the generic function, otherwise an error is signaled.

If {\it function-specifier\/} specifies a generic function that has a
different value for the {\bf :method-class} argument, the value is
changed, but any existing methods are not changed.

\label Syntax:

\Defun {ensure-generic-function} {function-specifier\/ \key \vtop{\hbox{:lambda-list}
\hbox{:argument-precedence-order}
\hbox{:declare}
\hbox{:documentation}
\hbox{:generic-function-class}
\hbox{:method-combination}
\hbox{:method-class}
\hbox{:environment}}}

\label Arguments:

The {\it function-specifier\/} argument is a symbol or a list of the
form {\tt (setf {\it symbol\/})}.

The keyword arguments correspond to the {\it option\/} arguments of
{\bf defgeneric}, except that the {\bf :method-class} and
and {\bf :generic-function-class} arguments can be class objects
as well as names.

The {\bf :environment\/} argument is the same as the {\bf
\&environment} argument to macro expansion functions.  It is typically
used to distinguish between compile-time and run-time environments.

\label Values:

The generic function object is returned.

%\label Remarks:

\label See Also:

{\bf defgeneric}

\endcom



\begincom{generic-flet}\ftype{Special Form}

\label Purpose:

The {\bf generic-flet} special form is analogous to the Common Lisp
{\bf flet} special form.  It produces new generic functions and
establishes new lexical function definition bindings.  Each generic
function is created with the set of methods specified by its method
descriptions.

The special form {\bf generic-flet} is used to define functions whose
names are meaningful only locally and to execute a series of forms
with these function definition bindings.  Any number of such local
generic functions may be defined.

The names of functions defined by {\bf generic-flet} have lexical
scope; they retain their local definitions only within the body of the
{\bf generic-flet}.  Any references within the body of the {\bf
generic-flet} to functions whose names are the same as those defined
within the {\bf generic-flet} are thus references to the local
functions instead of to any global functions of the same names.  The
scope of these generic function definition bindings, however, includes only
the body of {\bf generic-flet}, not the definitions themselves.
Within the method bodies, local function names that match those
being defined refer to global functions defined outside the {\bf
generic-flet}.  It is thus not possible to define recursive functions
with {\bf generic-flet}.

\label Syntax:

\Defspec {generic-flet} {
\vtop{\hbox{\lparen{\rm$\{$}\lparen function-specifier lambda-list
$\lbrack\!\lbrack\downarrow\!$ option $\vert$ \star{method-description} $\rbrack\!\rbrack$\rparen\star{\rm$\}$}\rparen}
\hbox{\star\form}}}
\Vskip 1pc!
{\it function-specifier\/}::$=$ \curly{{\it symbol\/} $\vert$ {\tt (setf {\it symbol\/})}}
\Vskip 1pc!
\settabs\+\hskip\leftskip&\cr
\+&{\it lambda-list\/}::$=$ (&\star{\curly{var}}  \cr
\+&&\ttbrac{{\opt} \star{\curly{var $\vert$ {\rm (}var\/{\rm )}}}}  \cr
\+&&\ttbrac{{\tt\&rest} {\it var\/}} \cr
\+&&{\tt [}{\key{}}&\star{\curly{var  $\vert$
{\rm (}\curly{var $\vert$ {\rm (}keyword var\/{\rm )}}{\rm )}}}\cr
\+&&&\brac{\tt\&allow-other-keys} {\tt ]} {\rm )} \cr
\Vskip 1pc!
{\it option\/}::$=$ {\tt\vtop{\hbox{(:argument-precedence-order
\plus{\curly{parameter-name}}) $\vert$}
\hbox{(declare \plus{\curly{declaration\/}}) $\vert$}
\hbox{(:documentation {\it string\/}) $\vert$}
\hbox{(:method-combination {\it symbol\/} \star{\curly{arg\/}}) $\vert$}
\hbox{(:generic-function-class {\it class-name\/}) $\vert$}
\hbox{(:method-class {\it class-name\/})}}}
%\Vskip 1pc!
\vfill\eject
{\it method-description\/}::$=$ {\tt (:method {\it method-specifier\/})}
\Vskip 1pc!
{\it method-specifier\/}::$=$ \vtop{\hbox{\it \star{\curly{method-qualifier\/}} specialized-lambda-list\/}
\hbox{\star{\curly{declaration\/ $\vert$ documentation\/}} \star{\curly{form\/}}}}

\label Arguments:

The {\it function-specifier\/}, {\it lambda-list\/}, {\it option}, {\it
method-qualifier}, and {\it specialized-lambda-list\/} arguments are
the same as for {\bf defgeneric}.

A {\bf generic-flet} local method definition is identical in form to the
method definition part of a {\bf defmethod}.

The body of each method is enclosed in an implicit block.  If {\it
function-specifier\/} is a symbol, this block bears the same name as
the generic function.  If {\it function-specifier\/} is a list of the
form {\tt (setf {\it symbol\/})}, the name of the block is {\it
symbol}.  The {\bf return-from} construct may be used to cause an exit
from the method as well as the block.

\label Values:

The result returned by {\bf generic-flet} is the value or values
returned by the last form executed.  If no forms are specified, 
{\bf generic-flet} returns {\bf nil}.

\label See Also:

{\bf generic-labels

defmethod

defgeneric

generic-function}

\endcom


\begincom{generic-function}\ftype{Special Form}

\label Purpose:

The {\bf generic-function} special form creates an anonymous generic
function. The generic function is created with the set of methods
specified by its method descriptions.

\label Syntax:

\Defspec {generic-function} {\vtop{\hbox{\it lambda-list}
%\hbox{\star{\curly{option $\vert$ method-description\/}}}}}
\hbox{$\lbrack\!\lbrack\downarrow\!$ option $\vert$ \star{method-description} $\rbrack\!\rbrack$}}}
\Vskip 1pc!
{\it option\/}::$=$ {\tt\vtop{\hbox{(:argument-precedence-order
\plus{\curly{parameter-name}}) $\vert$}
\hbox{(declare \plus{\curly{declaration\/}}) $\vert$}
\hbox{(:documentation {\it string\/}) $\vert$}
\hbox{(:method-combination {\it symbol\/} \star{\curly{arg\/}}) $\vert$}
\hbox{(:generic-function-class {\it class-name\/}) $\vert$}
\hbox{(:method-class {\it class-name\/})}}}
\Vskip 1pc!
{\it method-description\/}::$=$ {\tt (:method {\it method-specifier\/})}
\Vskip 1pc!
{\it method-specifier\/}::$=$ \vtop{\hbox{\it \star{\curly{method-qualifier\/}} specialized-lambda-list\/}
\hbox{\star{\curly{declaration\/ $\vert$ documentation\/}} \star{\curly{form\/}}}}

\label Arguments:

The {\it option}, {\it method-qualifier}, and {\it
specialized-lambda-list\/} arguments are the same as for {\bf
defgeneric}.

\label Values:

The generic function object is returned as the result.

\label Remarks:

If no method descriptions are specified, an anonymous generic function with no
methods is created.

\vfill\eject
\label See Also:

{\bf defgeneric

generic-flet

generic-labels

defmethod}

\endcom



\begincom{generic-labels}\ftype{Special Form}

\label Purpose:

The {\bf generic-labels} special form is analogous to the Common Lisp
{\bf labels} special form.  It produces new generic functions and
establishes new lexical function definition bindings.  Each generic
function is created with the set of methods specified by its method
descriptions.

The special form {\bf generic-labels} is used to define functions
whose names are meaningful only locally and to execute a series of
forms with these function definition bindings.  Any number of
such  local functions may be defined.  

The names of functions defined by {\bf generic-labels} have lexical
scope; they retain their local definitions only within the body of the
{\bf generic-labels} construct.  Any references within the body of the
{\bf generic-labels} construct to functions whose names are the same
as those defined within the {\bf generic-labels} form are thus
references to the local functions instead of to any global functions
of the same names.  The scope of these generic function definition bindings
includes the method bodies themselves as well as the body of the {\bf
generic-labels} construct.

\label Syntax:

\Defspec {generic-labels} {
\vtop{\hbox{\lparen{\rm$\{$}\lparen function-specifier lambda-list}
\hbox{\ \ \ $\lbrack\!\lbrack\downarrow\! $option $\vert$ \star{method-description}$\rbrack\!\rbrack$\rparen\star{\rm$\}$}\rparen}
\hbox{\star\form}}}
\Vskip 1pc!
{\it function-specifier\/}::$=$ \curly{{\it symbol\/} $\vert$ {\tt (setf {\it symbol\/})}}
\Vskip 1pc!
\settabs\+\hskip\leftskip&\cr
\+&{\it lambda-list\/}::$=$ (&\star{\curly{var}}  \cr
\+&&\ttbrac{{\opt} \star{\curly{var $\vert$ {\rm (}var\/{\rm )}}}}  \cr
\+&&\ttbrac{{\tt\&rest} {\it var\/}} \cr
\+&&{\tt [}{\key{}}&\star{\curly{var  $\vert$
{\rm (}\curly{var $\vert$ {\rm (}keyword var\/{\rm )}}{\rm )}}}\cr
\+&&&\brac{\tt\&allow-other-keys} {\tt ]} {\rm )} \cr
\Vskip 1pc!
{\it option\/}::$=$ {\tt\vtop{\hbox{(:argument-precedence-order
\plus{\curly{parameter-name}}) $\vert$}
\hbox{(declare \plus{\curly{declaration\/}}) $\vert$}
\hbox{(:documentation {\it string\/}) $\vert$}
\hbox{(:method-combination {\it symbol\/} \star{\curly{arg\/}}) $\vert$}
\hbox{(:generic-function-class {\it class-name\/}) $\vert$}
\hbox{(:method-class {\it class-name\/})}}}
\Vskip 1pc!
{\it method-description\/}::$=$ {\tt (:method {\it method-specifier\/})}
\Vskip 1pc!
{\it method-specifier\/}::$=$ \vtop{\hbox{\it \star{\curly{method-qualifier\/}} specialized-lambda-list\/}
\hbox{\star{\curly{declaration\/ $\vert$ documentation\/}} \star{\curly{form\/}}}}

\label Arguments:

The {\it function-specifier\/}, {\it lambda-list\/}, {\it option}, {\it
method-qualifier}, and {\it specialized-lambda-list\/} arguments are
the same as for {\bf defgeneric}.

A {\bf generic-labels} local method definition is identical in form to the
method definition part of a {\bf defmethod}.

The body of each method is enclosed in an implicit block.  If {\it
function-specifier\/} is a symbol, this block bears the same name as
the generic function.  If {\it function-specifier\/} is a list of the
form {\tt (setf {\it symbol\/})}, the name of the block is {\it
symbol}.  The {\bf return-from} construct may be used to cause an exit
from the method as well as the block.

\label Values:

The result returned by {\bf generic-labels} is the value or values
returned by the last form executed.  If no forms are specified, 
{\bf generic-labels} returns {\bf nil}.

\label See Also:

{\bf generic-flet

defmethod

defgeneric

generic-function}

\endcom


\begincom{get-method}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf get-method} takes a generic function and returns the
method object that has the given method qualifiers
and parameter specializers.   

\label Syntax:

\Defgen {get-method} {generic-function method-qualifiers specializers {\opt} errorp}

\label Method Signatures:

\Defmeth {get-method} {
\vtop{\hbox{({\it generic-function\/} standard-generic-function)}
\hbox{\it method-qualifiers specializers {\opt} errorp}}}

\label Arguments:

The {\it generic-function\/} argument is a generic function.

The {\it method-qualifiers\/} argument is a list of the
method qualifiers for the method.   The order of the method qualifiers
is significant.  

The {\it specializers\/} argument is a list of the parameter
specializers for the method.  It must correspond in length to
the number of required arguments of the generic function, or
else an error is signaled.  This means that to obtain the
default method on a given generic function, a list whose
elements are the class named {\bf t} must be given.

If there is no such method and the {\it errorp\/} argument is
unsupplied or is non-{\bf nil}, {\bf get-method} signals an error.
If there is no such method and the {\it errorp\/} argument is
{\bf nil}, {\bf get-method} returns {\bf nil}.  The default value of
{\it errorp\/} is {\bf t}.

\label Values:

The result of {\bf get-method} is the method object with the given
method qualifiers and parameter specializers.

\label Remarks:

The method qualifiers and parameter specializers are tested for equality
using {\bf equal}.

\endcom


\begincom{initialize-instance}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf initialize-instance} is called by {\bf
make-instance} to initialize a newly-created instance.  The function
{\bf initialize-instance} is called with the new instance, with the
initialization arguments that were supplied to {\bf make-instance},
and with any default initialization arguments.

The function {\bf initialize-instance} initializes the slots with
values according to the initialization argument list.  It behaves as
follows on each slot, whether shared or local:

\beginlist

\item{\bull} If an initialization argument in the defaulted initialization
argument list specifies a value for that slot, that value is stored into
the slot.

\item{\bull} Otherwise, if the slot is unbound and has an {\bf
:initform} form, that form is evaluated and the result is stored into the
slot.

\item{\bull} The rules mentioned in the section
``Rules for Duplicate Initialization Arguments'' are obeyed.

\endlist

\label Syntax:

\Defgen {initialize-instance} {instance \key {\tt \&allow-other-keys}}

\label Method Signatures:

\Defmeth {initialize-instance} {({\it instance\/} standard-object) \key  \&allow-other-keys}

\Defmeth {initialize-instance} {({\it class\/} standard-class) 
\&key \vtop{\hbox{:direct-superclasses}
\hbox{:direct-slots}
\hbox{:options}}}

\Defmeth {initialize-instance}
{\vtop{\hbox{({\it generic-function\/}}
\hbox{\&key \vtop{\rlap{:lambda-list}
                  \rlap{:argument-precedence-order}
                  \rlap{:declare}
                  \rlap{:documentation}
                  \rlap{:method-combination}
                  \rlap{:method-class}}}}\  standard-generic-function)}

\Defmeth {initialize-instance} {({\it method\/} standard-method) 
\key \vtop{\hbox{:lambda-list}
\hbox{:qualifiers}
\hbox{:specializers}
\hbox{:function}}}

\label Arguments:

The {\it instance\/} argument is the object to be initialized.

The {\bf initialize-instance} protocol may be used to initialize a class
object that is an instance of class {\bf standard-class} by
supplying the following initialization arguments: {\bf :direct-superclasses},
{\bf :direct-slots}, {\bf :options}.

\beginlist

\item{\bull}
The {\bf :direct-superclasses} argument is a list of class names or
class objects.

\item{\bull}
The {\bf :direct-slots} argument is a list of slot description objects.

\item{\bull}
The {\bf :options} argument is a list of class options of the type
accepted by {\bf defclass}, except that {\bf :metaclass} is not allowed.

\endlist

The {\bf initialize-instance} protocol may be used to initialize a
generic function object that is an instance of class {\bf
standard-generic-function} by supplying the following initialization
arguments: {\bf :lambda-list}, {\bf :argument-precedence-order}, {\bf
:declare}, {\bf :documentation}, {\bf :method-combination}, {\bf
:method-class}.

\beginlist

\item{\bull}
The {\bf :lambda-list} argument is a lambda-list of the type that may be
given to {\bf defgeneric}.   

\item{\bull}
The following arguments have the same semantics as the corresponding
arguments of {\bf defgeneric}, although their syntax may differ:

\itemitem{--}
The {\bf :argument-precedence-order} argument is a list containing the
parameter names for all required arguments.  
Each required argument must be included exactly once 
so that the full and unambiguous precedence order is 
supplied.   If this condition is not met, an error is signaled.

\itemitem{--}
The {\bf :method-combination} argument is a symbol or a list.  If it
is a symbol, that symbol names a type of method combination.  If it is
a list, its first element is a symbol that names a type of method
combination, and its remaining elements are any arguments accepted by
the method combination type.  Any arguments that follow that symbol
depend on the type of method combination.  Note that the standard
method combination type does not support any arguments.  However, all
types of method combination defined by the short form of {\bf
define-method-combination} accept an optional argument named {\it
order\/}, which defaults to {\bf :most-specific-first}, where a value of {\bf
:most-specific-last} reverses the order of the primary methods
without affecting the order of the auxiliary methods.

\itemitem{--}
The {\bf :documentation} argument is a string.

\itemitem{--}
The {\bf :declare} argument is a list of declaration specifiers.

\itemitem{--}
The {\bf :method-class} argument is a class or the name of a class.
\endlist

The {\bf initialize-instance} protocol may be used to initialize a method
object that is an instance of class {\bf standard-method} by
supplying the following initialization arguments: {\bf :lambda-list}
{\bf :qualifiers}, {\bf :specializers}, {\bf :function}.

\beginlist

\item{\bull}
The {\bf :lambda-list} argument is a specialized lambda-list of the
type that may be given to {\bf defmethod}.  The only information that
is used from this is that which is needed to construct a lambda-list
for {\bf defgeneric}.

\item{\bull}
The {\bf :qualifiers} argument is a list of the method
qualifiers for the method.

\item{\bull}
The {\bf :specializers} argument is a list of the parameter
specializers for the method.

\item{\bull}
The {\bf :function} argument is the method function.  If the length of
the list of specializers is not equal to the number of required
arguments of the method function, the result is undefined.
\endlist

\label Values:

The modified instance is returned as the result.

\label Remarks:

The keyword name of each keyword parameter specifier in the lambda-list
of any method defined on {\bf initialize-instance} becomes an initialization
argument for all classes for which that method is applicable.

Methods on {\bf initialize-instance} can be defined to implement
additional initialization arguments.  These methods are typically
{\bf :after} methods, although that is not a requirement.

Implementations are permitted to optimize {\bf :initform} forms that
neither produce nor depend on side-effects, by evaluating these forms
and storing them into slots before running any {\bf
initialize-instance} methods, rather than by handling them in the primary
{\bf initialize-instance} method.  (This optimization might be
implemented by having the {\bf allocate-instance} method copy a
prototype instance.)

Implementations are permitted to optimize default initial value forms
for initialization arguments associated with slots by not actually
creating the complete initialization argument list when the only
method that would receive the complete list is the method on {\bf
standard-object}.  In this case default initial value forms can be
treated like {\bf :initform} forms.  This optimization has no visible
effects other than a performance improvement.

\vfill\eject
\label See Also:

``Object Creation and Initialization''

{\bf make-instance

slot-boundp

slot-makunbound}

\endcom



\begincom{invalid-method-error}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf invalid-method-error} reports an applicable method
whose qualifiers are not valid for the method combination type.  The 
error message is constructed by using a format string and any arguments to it. 
Because an implementation may need to add additional contextual information 
to the error message, {\bf invalid-method-error} should be called
only within the dynamic extent of a method-combination function.

Whether {\bf invalid-method-error} returns to its caller or exits via
{\bf throw} is implementation dependent.

\label Syntax:

\Defgen {invalid-method-error} {method format-string {\rest} args}

\label Method Signatures:

\Defmeth {invalid-method-error} {({\it method\/} standard-method) {\it format-string\/} \&rest {\it args\/}}

\label Arguments:

The {\it method\/} argument is the invalid method object.  

The {\it format-string\/} argument is a control string that can be
given to {\bf format}, and {\it args\/} are any arguments required by
that string.

\label Remarks:

The function {\bf invalid-method-error} is called automatically when a
method fails to satisfy every qualifier pattern and predicate in a
{\bf define-method-combination} form. 
A method combination function that 
imposes additional restrictions should call {\bf invalid-method-error}
explicitly if it encounters a method it cannot accept.

The function {\bf invalid-method-error} uses the
condition-signaling system.

\label See Also: 

{\bf define-method-combination}

\endcom


\begincom{make-instance}\ftype{Standard Generic Function}

\label Purpose:

The function {\bf make-instance} creates and returns a new instance of
the given class.

\label Syntax:

\Defgen {make-instance} {class\/ {\tt \&rest {\it initargs}}}

\label Method Signatures:

\Defmeth {make-instance} {({\it class\/} standard-class) \&rest {\it initargs}}

\Defmeth {make-instance} {({\it generic-function\/} standard-generic-function)
\&rest {\it initargs}}

\Defmeth {make-instance} {({\it method\/} standard-method) 
\&rest {\it initargs}}

\Defmeth {make-instance} {({\it class\/} symbol) \&rest {\it initargs}}

\label Arguments:

The {\bf make-instance} protocol may be used as described in the
section ``Object Creation and Initialization'' to create instances of
classes. The {\it class\/} argument is a class object or a symbol that
names a class.  The remaining arguments are a list of alternating
initialization argument names and values.

If the last of the above methods is selected, that method invokes
{\bf make-instance} on the arguments {\tt (symbol-class {\it class\/})} and
{\it initargs}.

The initialization arguments are checked within {\bf make-instance}
by the generic function {\bf check-initargs}.
See the section ``Object Creation and Initialization.''

The {\bf make-instance} protocol may be used to create a generic function
object that is an instance of class {\bf standard-generic-function} by
supplying the following initialization arguments: {\bf :lambda-list},
{\bf :argument-precedence-order}, {\bf :declare}, {\bf :documentation},
{\bf :method-combination}, {\bf :method-class}.

The {\bf make-instance} protocol may be used to create a method
object that is an instance of class {\bf standard-method} by
supplying the following initialization arguments: {\bf :lambda-list}
{\bf :qualifiers}, {\bf :specializers}, {\bf :function}.

For a description of the use of these arguments, see {\bf initialize-instance}.

\label Values:

The new instance is returned.

\label Remarks:

The function {\bf class-of} can be used to determine the class of the
instance that is returned.

The meta-object protocol can be used to define new methods on {\bf
make-instance} to replace the object-creation protocol.

\label See Also:

``Object Creation and Initialization''

{\bf defclass

initialize-instance

check-initargs

class-of

defgeneric

defmethod}

\endcom




\begincom{make-instances-obsolete}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf make-instances-obsolete} is invoked
automatically by the system when {\bf defclass} has been used to
redefine an existing standard class and the set of local slots accessible in an
instance is changed or the order of slots in storage is changed.  It
can also be explicitly invoked by the user.

The function {\bf make-instances-obsolete} has the effect of
initiating the process of updating the instances of the
newly-redefined class. During updating, the generic function {\bf
update-instance-structure} may be invoked.

\label Syntax:

\Defgen {make-instances-obsolete} {class}

\label Method Signatures:

\Defmeth {make-instances-obsolete} {({\it class\/} standard-class)}

\Defmeth {make-instances-obsolete} {({\it symbol\/} symbol)}

\label Arguments:

The {\it class\/} argument is a class object or a symbol that names
the class whose instances are to be made obsolete.

If the second of the above methods is selected, that method invokes
{\bf make-instances-obsolete} on {\it symbol\/} and {\tt
(symbol-class {\it symbol\/})}.

\label Values:

The modified class is returned.  The result of {\bf make-instances-obsolete}
is {\bf eq} to the {\it class} argument supplied to the first of the above
methods.

\label See Also:

{\bf update-instance-structure}

``Redefining Classes''

\endcom



\begincom{make-method-call}\ftype{Function}

\label Purpose:

The function {\bf make-method-call} is used in method combination.  It
has dynamic scope within the body of a {\bf define-method-combination}
form.

The function {\bf make-method-call} returns a form whose effect is the
same as a form whose first element is the operator specified by the
{\bf :operator} keyword argument (the default is {\bf progn}) and the
rest of which is a list of forms that call the methods in the given
method list, except when the operator is {\bf :call-next-method}.
Each method receives the same arguments that the generic function
received.  The function {\bf make-method-call} hides the
implementation-dependent details of how methods are called.

\label Syntax:

\Defun {make-method-call} {method-list\/ {\key :operator :identity-with-one-argument}}

\label Arguments:

Each element of {\it method-list\/} can be either a method object or a list.
When a list is given, it is regarded as a form and converted when necessary
into a method whose body is that form.

If the value of {\bf :identity-with-one-argument} is true and {\it
method-list\/} contains exactly one element, the result is simply a
form that calls that single method and does not invoke the
operator.  If {\bf :operator} is {\bf progn}, the default for {\bf
:identity-with-one-argument} is true; otherwise the default for this
option is false.  This option is to be used with operators that are
identity operators when applied to one argument, that is, such
operators as {\bf progn}, {\bf and}, {\bf $+$}, and {\bf max}.  This
optimization can enable the use of an existing method as the
effective method, thus avoiding the need to create a new effective method.

If {\it method-list\/} is {\bf nil}, the result is a call to
the specified operator with no arguments or a form with the same effect.

If {\bf :operator} is {\bf :call-next-method}, the result is a form
that calls the first method, $M\sub 1$, and arranges for {\bf
call-next-method} to reach the next method, $M\sub 2$, in the list.
An invocation of {\bf call-next-method} in $M\sub 2$ reaches the
following method, and this continues through the methods in the order
they appear in {\it method-list}.  If {\bf call-next-method} is
invoked by the last method in {\it method-list}, an error is signaled.
If {\it method-list\/} is {\bf nil} and {\bf :operator} is {\bf
:call-next-method}, the result is {\bf nil}.

As a convenience, if {\it method-list\/} is a method object, it is
automatically converted to a one-element list of that method.

\eject
\label Values:

The result is a form whose effect is the same as a form whose first
element is the operator specified by the {\bf :operator} keyword
argument and the rest of which is a list of forms that call the
methods in {\it method-list}.

\label See Also: 

{\bf define-method-combination}

\endcom


\begincom{method-combination-error}\ftype{Function}

\label Purpose:

The function {\bf method-combination-error} reports a problem in
method combination.  The error message is constructed by using a format
string and any arguments to it.  Because an implementation may need to
add additional contextual information to the error message, {\bf
method-combination-error} should be called only within the dynamic
extent of a method combination function.

Whether {\bf method-combination-error} returns to its caller or exits
via {\bf throw} is implementation dependent.

\label Syntax:

\Defun {method-combination-error} {format-string {\rest} args}

\label Arguments: 

The {\it format-string\/} argument is a control string that can be
given to {\bf format}, and {\it args\/} are any arguments required by
that string.

\label Remarks:

The function {\bf method-combination-error} signals an error.

\label See Also: 

{\bf define-method-combination}

\endcom




\begincom{method-qualifiers}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf method-qualifiers} returns a list of the
qualifiers of the given method.

\label Syntax:

\Defgen {method-qualifiers} {method}

\label Method Signatures:

\Defmeth {method-qualifiers} {({\it method\/} standard-method)}

\label Arguments:

The {\it method\/} argument is a method object. 

\label Values:

A list of the qualifiers of the given method is returned.

\label Examples:

\screen!

(setq methods (remove-duplicates methods
                                 :from-end t
                                 :key #'method-qualifiers
                                 :test #'equal))

\endscreen!

\label See Also: 

{\bf define-method-combination}

\endcom

 

\begincom{next-method-p}\ftype{Function}

\label Purpose:

The locally defined function {\bf next-method-p} can be used within
the body of a method defined by a method-defining form to determine
whether a next method exists.

\label Syntax:

\Defun {next-method-p} {}

\label Arguments:

The function {\bf next-method-p} takes no arguments.

\label Values:

The function {\bf next-method-p} returns true or false.

\label Remarks:

Like {\bf call-next-method}, the function {\bf next-method-p} has 
lexical scope and indefinite extent.
%  The lexical scope includes any
%default initial value forms that appear in the method's lambda-list.

\label See Also:

{\bf call-next-method}

\endcom


\begincom{no-applicable-method}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf no-applicable-method} is called when a
generic function is invoked and no method on that generic function is
applicable.

The generic function {\bf no-applicable-method} is not intended to be called
by programmers.  Programmers are expected to write methods for it.

\label Syntax:

\Defgen {no-applicable-method} {generic-function {\rest} function-arguments}

\label Method Signatures:

\Defmeth {no-applicable-method} {\vtop{\hbox{({\it generic-function\/} standard-generic-function)}
\hbox{\&rest {\it function-arguments\/}}}}

\label Arguments:

The {\it generic-function\/} argument of {\bf no-applicable-method} is the
generic function object on which no applicable method was found.  

The {\it function-arguments} argument is a list of the arguments to that
generic function.

\label Values:

The method on {\bf standard-generic-function} signals an
error.

\endcom


\begincom{print-object}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf print-object} writes the printed
representation of an object to a stream.  The function {\bf
print-object} is called by the print system; it should not be called
by the user.

Each implementation is required to provide a default method on {\bf 
print-object}, that is, a method on the class {\bf t}.  Implementations 
are free to add methods on specific classes.  Users can write methods 
for {\bf print-object} for their own classes if they do not wish to 
inherit an implementation-supplied method. 

\label Syntax:

\Defgen {print-object} {object stream}

\label Method Signatures:

\Defmeth {print-object} {({\it object\/} t) {\it stream\/}}

\label Arguments:

The first argument is any Lisp object.   The second argument is a
stream; it cannot be {\bf t} or {\bf nil}. 

\label Values:

The function {\bf print-object} returns its first argument, the object.  

\label Remarks:

Methods on {\bf print-object} must obey the print control special
variables described in {\it Common Lisp: The Language}.  The
specific details are the following:

\beginlist

\item{\bull}
Each method must implement {\bf *print-escape*}. 

\item{\bull} 
The {\bf *print-pretty*} control variable can be ignored
by most methods other than the one for lists.

\item{\bull}
The {\bf *print-circle*} control variable is handled by the printer
and can be ignored by methods.

\item{\bull}
The printer takes care of {\bf *print-level*} automatically, provided that
each method handles exactly one level of structure and
calls {\bf write} (or an equivalent function) recursively if
there are more structural levels.  The printer's decision
of whether an object has components (and therefore should
not be printed when the printing depth is not less than
{\bf *print-level*}) is implementation dependent.  In some
implementations its {\bf print-object} method is not called; in
others the method is called, and the determination that the
object has components is based on what it tries to write
to the stream.

\item{\bull}
Methods that produce output of indefinite length must obey
{\bf *print-length*}, but most methods other than the one for lists can
ignore it.

\item{\bull}
The {\bf *print-base*}, {\bf *print-radix*}, {\bf *print-case*}, {\bf
*print-gensym*}, and {\bf *print-array*} control variables apply
to specific types of objects and are handled by the methods for those
objects.

\endlist

If these rules are not obeyed, the results are undefined.

In general, the printer and the {\bf print-object} methods should not
rebind the print control variables as they operate recursively through the
structure, but this is implementation dependent.

In some implementations the stream argument passed to a {\bf
print-object} method is not the original stream, but is an
intermediate stream that implements part of the printer.  Methods
should therefore not depend on the identity of this stream.

All of the existing printing functions ({\bf write}, {\bf prin1}, {\bf
print}, {\bf princ}, {\bf pprint}, {\bf write-to-string}, {\bf
prin1-to-string}, {\bf princ-to-string}, the {\tt ~}{\bf S} and {\tt
~}{\bf A} format operations, and the {\tt ~}{\bf B}, {\tt ~}{\bf D},
{\tt ~}{\bf E}, {\tt ~}{\bf F}, {\tt ~}{\bf G}, {\tt ~}{\bf \$}, {\tt
~}{\bf O}, {\tt ~}{\bf R}, and {\tt ~}{\bf X} format operations when they
encounter a non-numeric value) are required to be changed to go
through the {\bf print-object} generic function.  Each implementation is
required to replace its former implementation of printing with one or
more {\bf print-object} methods.  Exactly which classes have methods for
{\bf print-object} is not specified; it would be valid for an implementation
to have one default method that is inherited by all system-defined
classes.

\endcom




\begincom{remove-method}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf remove-method} removes a method from a
generic function.  It destructively modifies the specified generic
function and returns the modified generic function as its result.

\label Syntax:

\Defgen {remove-method} {generic-function method}

\label Method Signatures:

\Defmeth {remove-method} {\vtop{\hbox{({\it generic-function\/} standard-generic-function)}
\hbox{({\it method\/} standard-method)}}}

\label Arguments:

The {\it generic-function\/} argument is a generic function
object.

The {\it method\/} argument is a method object.  The function {\bf
remove-method} does not signal an error if the method is not one of the
methods on the generic function.

\label Values:

The modified generic function is returned.  The result of {\bf remove-method} 
is {\bf eq} to the {\it generic-function\/} argument.

\label See Also:

{\bf
get-method

add-method}

\endcom


\begincom{slot-boundp}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf slot-boundp} tests whether a specific slot in an
instance is bound.

\label Syntax:

\Defgen {slot-boundp} {instance slot-name}

\label Method Signatures:

\Defmeth {slot-boundp} {({\it instance\/} standard-object) {\it slot-name\/}}

\label Arguments:

The arguments are the instance and the name of the slot.

\label Values:

The function {\bf slot-boundp} returns true or false.

\label Remarks:

The generic function {\bf slot-boundp} allows for writing {\bf :after}
methods on {\bf initialize-instance} in order to initialize only
those slots that have not already been bound.

If no slot of the given name exists in the instance, {\bf slot-missing}
is called as follows:\hfil\break
{\tt (slot-missing (class-of {\it instance\/})
{\it instance slot-name\/} 'slot-boundp)}.

The function {\bf slot-boundp} is implemented using {\bf
slot-boundp-using-class}.

\label See Also:

{\bf slot-missing}

\endcom



\begincom{slot-exists-p}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf slot-exists-p} tests whether the specified object has
a slot of the given name.

\label Syntax:

\Defgen {slot-exists-p} {object slot-name}

\label Method Signatures:

\Defmeth {slot-exists-p} {({\it object\/} standard-object) {\it slot-name\/}}

\label Arguments:

The {\it object\/} argument is any object.  The {\it slot-name\/} argument
is a symbol.

\label Values:

The function {\bf slot-exists-p} returns true or false.

\endcom


\begincom{slot-makunbound}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf slot-makunbound} restores a slot in an instance to
the unbound state.

\label Syntax:

\Defgen {slot-makunbound} {instance slot-name}

\label Method Signatures:

\Defmeth {slot-makunbound} {({\it instance\/} standard-object) {\it slot-name\/}}

\label Arguments:

The arguments to {\bf slot-makunbound} are the instance and the name of
the slot.

\label Values:

The instance is returned as the result.

\label Remarks:

If no slot of the given name exists in the instance, {\bf slot-missing}
is called as follows:\hfil\break
{\tt (slot-missing (class-of {\it instance\/}) 
{\it instance slot-name\/} 'slot-makunbound)}.

The function {\bf slot-makunbound} is implemented using {\bf
slot-makunbound-using-class}.

\label See Also:

{\bf slot-missing}

\endcom




\begincom{slot-missing}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf slot-missing} is invoked when an attempt is
made to access a slot in an object and the name of the slot provided
is not a name of a slot in that class.

The generic function {\bf slot-missing} is not intended to be called by
programmers.  Programmers are expected to write methods for it.

The method on {\bf standard-class} signals an error.

\label Syntax:

\Defgen {slot-missing} {class object slot-name operation {\opt} new-value}

\label Method Signatures:

\Defmeth {slot-missing} {\vtop{\hbox{({\it class\/} standard-class) {\it object slot-name}}
\hbox{{\it operation\/} {\opt} new-value}}}

\label Arguments:

The required arguments to {\bf slot-missing} are the class of the object
that is being accessed, the object, the slot name, and a symbol that
indicates the operation that caused  {\bf slot-missing} to be invoked.
The optional argument to {\bf slot-missing} is used when the operation
is attempting to set the value of the slot.

\label Values:

The method on {\bf standard-class} signals an error.

If a method written for {\bf slot-missing} returns values, these
values get returned as the values of the original function invocation.

\label Remarks:

The generic function {\bf slot-missing} may be called during
evaluation of {\bf slot-value}, {\bf (setf slot-value)}, 
{\bf slot-boundp}, and {\bf slot-makunbound}.  For each
of these operations the corresponding symbol for the {\it operation\/}
argument is {\bf slot-value}, {\bf setf}, {\bf slot-boundp}, {\bf
slot-makunbound}.  

The set of arguments (including the class of the instance) facilitates
defining methods on the metaclass for {\bf slot-missing}.

\endcom


\begincom{slot-unbound}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf slot-unbound} is called when an
unbound slot is read in an instance whose metaclass is {\bf
standard-class}.

The generic function {\bf slot-unbound} is not intended to be called
by programmers.  Programmers are expected to write methods for it.
The function {\bf slot-unbound} is called only by the function
{\bf slot-value-using-class} and thus indirectly by {\bf slot-value}.

\label Syntax:

\Defgen {slot-unbound} {class instance slot-name}

\label Method Signatures:

\Defmeth {slot-unbound} {({\it class\/} standard-class) {\it instance slot-name}}

\label Arguments:

The arguments to {\bf slot-unbound} are the class of the instance
whose slot was accessed, the instance itself, and the name of the
slot.

\label Values:

The method on {\bf standard-class} signals an error.

If a method written for {\bf slot-unbound} returns values, these
values get returned as the values of the original function invocation.

\label Remarks:

An unbound slot may occur if no {\bf :initform} form was
specified for the slot and the slot value has not been set, or if {\bf
slot-makunbound} has been called on the slot.

\label See Also:

{\bf slot-value-using-class

slot-makunbound}

\endcom


\begincom{slot-value}\ftype{Function}

\label Purpose:

The function {\bf slot-value} returns the value contained in the slot
{\it slot-name\/} of the given object.  If there is no slot with that
name, {\bf slot-missing} is called.  If the slot is unbound,
{\bf slot-unbound} is called.

The macro {\bf setf} can be used with {\bf slot-value} to change the value
of a slot. 

\label Syntax:

\Defun {slot-value} {object slot-name}

\label Arguments:

The arguments are the object and the name of the given slot.

\label Values:

The result is the value contained in the given slot.

\label Remarks:

If an attempt is made to read a slot and no slot of the given name
exists in the instance, {\bf slot-missing} is called as follows: {\tt
(slot-missing (class-of {\it instance\/}) {\it instance slot-name\/}
'slot-value)}.

If an attempt is made to write a slot and no slot of the given name
exists in the instance, {\bf slot-missing} is called as follows: {\tt
(slot-missing (class-of {\it instance\/}) {\it instance slot-name\/}
'(setf slot-value) {\it new-value\/})}.

The function {\bf slot-value} is implemented using {\bf
slot-value-using-class}.

Implementations may optimize {\bf slot-value} by compiling it inline.

\label See Also:

{\bf slot-missing}

{\bf slot-unbound}

\endcom



\begincom{symbol-class}\ftype{Function}

\label Purpose:

The function {\bf symbol-class} returns the class object named by the
given symbol in the given environment.  If there is no class named by
the given symbol, {\bf symbol-class} signals an error.

\label Syntax:

\Defun {symbol-class} {symbol {\opt} environment}

\label Arguments:

The first argument to {\bf symbol-class} is a symbol. 

The optional {\it environment\/} argument is the same as the {\bf
\&environment} argument to macro expansion functions.  It is typically
used to distinguish between compile-time and run-time environments.

If {\it environment\/} is not specified, the environment is the null
environment.

\label Values:

The result of {\bf symbol-class} is the class object named by the given symbol.

\label Remarks:

The class associated with a particular symbol can be changed by using
{\bf setf} with {\bf symbol-class}.  The results are undefined if
the user attempts to change the class associated with a symbol that is
defined as a type specifier by {\it Common Lisp: The Language.}

\label See Also:

{\bf cboundp

cmakunbound}

\endcom


\begincom{symbol-macrolet}\ftype{Macro}

\label Purpose:

The macro {\bf symbol-macrolet} allows the substitution of forms for
variable names within a lexical scope.

\label Syntax:

\Defmac {symbol-macrolet} {\paren{\star{\paren{symbol expansion\/}}} {\body} {\it body\/}}

\label Arguments:

The {\it symbol\/} argument specifies the symbol with which the form specified
by the {\it expansion\/} argument is to be associated.

\label Values:

The result returned is that obtained by executing the forms specified
by the {\it body\/} argument.

\label Remarks:

The lexical scope of {\bf symbol-macrolet} is {\it body\/}; it does
not include {\it expansion}.

Each reference to {\it symbol\/} as a variable within the lexical scope
of {\bf symbol-macrolet} is replaced by {\it expansion\/} (not the
result of evaluating {\it expansion\/}).

The macro {\bf symbol-macrolet} is the basic mechanism that is used to
implement {\bf with-slots}.

When the body of the {\bf symbol-macrolet} form is expanded, any use of
{\bf setq} to set the value of one of the specified variables is
converted to a use of {\bf setf}.

\label See Also:

{\bf with-slots}

\endcom

\begincom{update-instance-structure}\ftype{Standard Generic Function}

\label Purpose:

The generic function {\bf update-instance-structure} is not intended to
be called by programmers. Programmers are expected to write methods for it.
The generic function {\bf update-instance-structure} is called
by the mechanism activated by {\bf make-instances-obsolete}.

The function {\bf update-instance-structure} initializes the
newly-added slots of an instance when a class has been redefined.
Each local slot with no slot by the same name in the old version of
the class is initialized to the value of the corresponding {\bf
:initform} option of the new class or remains unbound if the new
version of the class does not specify or inherit the {\bf :initform}
option for that slot.  The {\bf :initform} form is evalued in the
lexical environment of its defining {\bf defclass} form.

\label Syntax:

\Defgen {update-instance-structure} {instance added-slots deleted-slots property-list}

\label Method Signatures:

\Defmeth {update-instance-structure} {\vtop{\hbox{({\it instance\/} standard-object)}
\hbox{\it added-slots deleted-slots property-list}}}

\label Arguments:

When {\bf make-instances-obsolete} is invoked or when a class has been
redefined and an instance is being updated, a property list is created
that captures the slot names and values of all the deleted slots with
values in the original instance.  The structure of the instance is
transformed so that it conforms to the current class definition.  The
arguments to {\bf update-instance-structure} are this transformed
instance, a list of the new slots added to the instance, a list of the
old slots deleted from the instance, and the property list containing
the slot names and values for slots that were deleted and had values.

\label Values:

The value returned by {\bf update-instance-structure} is ignored.

\label Examples:

\screen!

(defclass position () ())

(defclass x-y-position (position)
    ((x :initform 0 :accessor position-x)
     (y :initform 0 :accessor position-y)))

;;; It turns out we use polar co-ordinates more than Cartesian 
;;; co-ordinates, so we alter the representation and add some
;;; new accessor methods.

(defmethod update-instance-structure
   ((pos x-y-position) added deleted plist)
  ;; Copy the position information from old to new to make new
  ;; be a rho-theta-position at the same position as old.
  (let ((x (getf plist 'x))
        (y (getf plist 'y)))
    (setf (position-rho new) (sqrt (+ (* x x) (* y y)))
          (position-theta new) (atan y x))))

(defclass x-y-position (position)
    ((rho :initform 0 :accessor position-rho)
     (theta :initform 0 :accessor position-theta)))

;;; All instances of the old x-y-position class will be updated
;;; automatically.

;;; We give the new representation the look and feel of the old one

(defmethod position-x ((pos x-y-position))  
 (with-slots pos (rho theta) (* rho (cos theta))))

(defmethod (setf position-x) (new-x (pos x-y-position))
 (with-slots pos (rho theta)
  (let ((y (position-y pos)))
    (setq rho (sqrt (+ (* new-x new-x) (* y y)))
          theta (atan y new-x))
    new-x)))

(defmethod position-y ((pos x-y-position))
 (with-slots pos (rho theta) (* rho (sin theta))))

(defmethod (setf position-y) (new-y (pos x-y-position))
 (with-slots pos (rho theta)
  (let ((x (position-x pos)))
    (setq rho (sqrt (+ (* x x) (* new-y new-y)))
          theta (atan new-y x))
    new-y)))

\endscreen!

\vfill\eject
\label See Also:

``Redefining Classes''

{\bf make-instances-obsolete}

\endcom


\begincom{with-added-methods}\ftype{Special Form}

\label Purpose:

The {\bf with-added-methods} special form is an extension to the \OS\ {\bf
generic-labels}.  It produces new generic functions and establishes new
lexical function definition bindings.  Each generic function is created by
adding the set of methods specified by its method definitions to a copy of the
lexically visible generic function of the same name and its methods.  If
such a generic function does not already exist, a new generic function is
created; this generic function has lexical scope.

The special form {\bf with-added-methods} is used to define functions
whose names are meaningful only locally and to execute a series of
forms with these function definition bindings.  Any number of
such local functions may be defined.  

The names of functions defined by {\bf with-added-methods} have lexical
scope; they retain their local definitions only within the body of the
{\bf with-added-methods} construct.  Any references within the body of the
{\bf with-added-methods} construct to functions whose names are the same
as those defined within the {\bf with-added-methods} form are thus
references to the local functions instead of to any global functions
of the same names.  The scope of these generic function definition bindings
includes the method bodies themselves as well as the body of the {\bf
with-added-methods} construct.

\label Syntax:

\Defspec {with-added-methods} {
\vtop{\hbox{\lparen function-specifier lambda-list\/}
\hbox{\ $\lbrack\!\lbrack\downarrow\!$ option $\vert$ \star{method-description}\thinspace $\rbrack\!\rbrack$\thinspace\rparen}
\hbox{\star\form}}}
\Vskip 1pc!
{\it function-specifier\/}::$=$ \curly{{\it symbol\/} $\vert$ {\tt (setf {\it symbol\/})}}
\Vskip 1pc!
{\it option\/}::$=$ {\tt\vtop{\hbox{(:argument-precedence-order
\plus{\curly{parameter-name}}) $\vert$}
\hbox{(declare \plus{\curly{declaration\/}}) $\vert$}
\hbox{(:documentation {\it string\/}) $\vert$}
\hbox{(:method-combination {\it symbol\/} \star{\curly{arg\/}}) $\vert$}
\hbox{(:generic-function-class {\it class-name\/}) $\vert$}
\hbox{(:method-class {\it class-name\/})}}}
\Vskip 1pc!
{\it method-description\/}::$=$ {\tt (:method {\it method-specifier\/})}
\Vskip 1pc!
{\it method-specifier\/}::$=$ \vtop{\hbox{\it \star{\curly{method-qualifier\/}} specialized-lambda-list\/}
\hbox{\star{\curly{declaration\/ $\vert$ documentation\/}} \star{\curly{form\/}}}}

\vfill\eject
\label Arguments:

The {\it function-specifier\/}, {\it option}, {\it method-qualifier}, and {\it
specialized-lambda-list\/} arguments are the same as for \hfil\break
{\bf defgeneric}.

The body of each method is enclosed in an implicit block.  If
{\it function-specifier\/} is a symbol, this block bears the same name as the
generic function.  If {\it function-specifier\/} is a list of the form {\tt
(setf {\it symbol\/})}, the name of the block is {\it symbol}.  The {\bf
return-from} construct may be used to cause an exit from the method as
well as the block.

\label Values:

The result returned by {\bf with-added-methods} is the value or values
returned by the last form executed.  If no forms are specified, 
{\bf with-added-methods} returns {\bf nil}.

\label Remarks:

If a generic function with the given name already
exists, the lambda-list specified in the {\bf with-added-methods} form
must be congruent with the lambda-list of that function, or else an
error is signaled.

If a generic function with the given name already exists, the {\it
option\/} arguments are inherited from the existing generic function
as follows:

\beginlist

\item{\bull}
If {\it function-specifier\/} specifies a generic function that has a
different value for any of the following {\it option\/} arguments, the
copy of that generic function is modified to have the new value: {\bf
:argument-precedence-order}, {\bf declare}, {\bf :documentation}, {\bf
:generic-function-class}, {\bf :method-combination}.

\item{\bull}
If {\it function-specifier\/} specifies a generic function that has a
different value for the {\bf :method-class} {\it option\/} argument,
that value is changed in the copy of that generic function, but any
methods copied from the existing generic function are not changed.
\endlist

If a function of the given name already exists, that function is copied into
the default method for a generic function of the given name.  Note that
this behavior differs from that of {\bf defgeneric}.

If a macro or special form of the given name already exists, an error
is signaled.

If there is no existing generic function, the {\it option\/} arguments have
the same default values as the {\it option\/} arguments to {\bf defgeneric}.

\vfill\eject
\label See Also:

{\bf generic-labels

generic-flet

defmethod

defgeneric

ensure-generic-function}

\endcom


\begincom{with-slots}\ftype{Macro}

\label Purpose:

The macro {\bf with-slots} creates a lexical context for referring to
specified slots as though they were variables.  Within such a context
the value of the slot can be specified by using its slot name, as if
it were a lexically bound variable.  Both {\bf setf} and {\bf setq}
can be used to set the value of the slot.

The macro {\bf with-slots} translates an appearance of the slot name as
a variable into a call to {\bf slot-value}.

\label Syntax:  

\Defmac {with-slots} {instance-form\/ {\paren{\star{\curly{slot-entry\/}}}} {\body} body}

{\it slot-entry\/}::$=$ {\it slot-name $\vert$ {\paren{variable-name slot-name\/}}}

\label Examples:

\screen!

(with-slots position-1
            (x y)
  (sqrt (+ (* x x)
           (* y y))))


(with-slots position-1
            ((x1 x)
             (y1 y))
  (with-slots position-2
              ((x2 x)
               (y2 y))
     (psetf x1 x2
            y1 y2))))


  (with-slots position
              (x y)
    (setq x (1+ x)
          y (1+ y)))
\endscreen!

\vfill\eject
\label Remarks:

A {\bf with-slots} expression of the form:

$$\openup1\jot\vbox{\settabs\+\cr
\+{\tt (with-slots} {\it instance} (${\hbox{{\it slot-entry}}}\sub 1%
\ldots{\hbox{{\it slot-entry}}}\sub n$) ${\hbox{{\it form}}}\sub 1%
\ldots{\hbox{{\it form}}}\sub k$)\cr}$$

\noindent expands into the equivalent of

$$\openup1\jot\vbox{\settabs\+\cr
\+{\tt (}&{\tt let ((\#:temp} {\it instance}{\tt ))}\cr
\+&{\tt (symbol-macrolet (}${\hbox{{\it Q}}}\sub 1\ldots%
{\hbox{{\it Q}}}\sub n${\tt )} ${\hbox{{\it form}}}\sub 1%
\ldots{\hbox{{\it form}}}\sub k${\tt )}\cr}$$

\noindent where ${\hbox{{\it Q}}}\sub i$ is 

$$\vbox{\hbox{{\tt (}${\hbox{{\it slot-entry}}}\sub i$
{\tt (slot-value \#:temp} '${\hbox{{\it slot-entry}}}\sub i${\tt ))}}}$$

\noindent if ${\hbox{{\it slot-entry}}}\sub i$ is a symbol
and is

$${\vbox{\hbox{{\tt (}${\hbox{{\it variable-name}}}\sub i$ %
{\tt (slot-value \#:temp} '${\hbox{{\it slot-name}}}\sub i${\tt ))}}}}$$

\noindent if ${\hbox{{\it slot-entry}}}\sub i$
is of the form 

$$\vbox{\hbox{{\tt (}${\hbox{{\it variable-name}}}\sub i$ %
${\hbox{{\it slot-name}}}\sub i${\tt )}}}$$

\label See Also:

{\bf symbol-macrolet}

\endcom
\endChapter
\bye